I think you're basically right, secular people are using self-help books to get one of the things that religious people get from their religions.
This isn't the whole picture, though. Religion offers a lot beyond life advice and norm coordination. For many it also helps with finding meaning or purpose in life and with finding community. Self-help might touch on these a bit (things like processes for finding purpose or "a passion" and advice for how to make friends), but it doesn't in itself offer the broader containers religions often provide to offer those things.
Put this way, self-help is a bit like just doing something in the space that religion covers with texts and talks, but not covering its other functions.
(None of this is to say that religions don't sometimes do bad things, etc. etc., because I know someone will read my comment and be like "but what about this bad thing this religion did to me/others". Or that you can't live a fulfilling life without religion. But it's worth admitting that for many people, myself included, religion serves an important role in their lives, and it's worth considering how secular people meet those same needs that religious people meet via their religion.)
None of this is to say that religions don't sometimes do bad things
Furthermore, self-improvement has a lot of potential for harm too.
Morality is what prevents social animals from defecting in the one-shot PDs. Society is what prevents social animals from defecting in iterated PDs. So morality is an evolutionary adaptation. In higher animals like primates and whales, where culture is essential, morality is largely learned (including by reading books). In lower animals, like dogs and bees, it is largely innate. The cultural behavioral norms are malleable and depend on the society, thus theft/violence/murder can be acceptable in some cases and prohibited in others, there are no absolutes.
Everything else, like religion, is just elaborate fluff on top.
I'm not sure that is as sound as suggested. Just when are we really in a one-shot PD setting? In the, I might argue, rare cases where we are, is it really morality or merely habit of thinking that set our behavior?
Just when are we really in a one-shot PD setting?
Any situation where the defection would not be uncovered, can be treated as one-shot PD. Theft, slacking off, you name it.
Yes, but when is that really the case. Perhaps it's a case of my just not seeing the setting as that of pure isolation. So while we might call it morality perhaps it is potential impact in other games we also play that are not one-shot.
For instance, I see someone drop $10,000 (or $100 if you want) and not notice they did. I let them walk out of site and notice no one else is around and quietly pick it up and put it in my pocket.
Later I'm out with friend having lunch or drinks and offer to pick up the tab. In generally we all tend to pay our own way as none generally had a lot of money to just throw around. They start asking why so how do I explain?
Or perhaps my child has been asking for a special toy that was not in the budget. Suddenly I can buy it. How do I explain that to my spouse and child?
The idea being the lives we live is not one with the degree of separability implied by the one-shot assumption, so the game setting is really a repeated game but not always with same players.
Perhaps that is just explaining why morality might emerge and so your point holds but I'm not sure.
They start asking why so how do I explain?
Then it's no longer pure one-shot, I agree. But there are plenty of cases where defection can be done with impunity, and personal conscience is all that is keeping one from it. I doesn't have to be financial, either.
I agree with many of the suggestions here and in other comments. I want to note: another major thing filling religion's hole is fandom. And as much as I love (trans)humanist ritual, I think fandom might be the healthiest religious substitute by virtue of not looking plausibly-real, or getting in the way of other epistemic updates. (If you join the Blue / Red / Grey Tribe or Communist civic religions, you may have to contend with your views of economics or science being bound with your identity. But, if you join the Harry Potter Fandom, less so)
Most fans have a superficial relationship with their fandom. But, many go to meetups, conventions, forums and games that make up a lot of their social life. Meanwhile they create art that does at-least-sometimes rise to the level of religious inspiration.
Often I think they do get at least some morality from (Star Wars / Harry Potter / Buffy the Vampire Slayer).
(One of the most boggling moments was the Twitch Plays Pokemon phenomenon, which came with self-organized factions, music, and weird narrative)
So-called 'civil religions'. They are manifold and varied and spring up repeatedly across history. Now is no different.
Old-school 19th century nationalism flared as the Christian religion began really losing its grip. Modern political mythologies hinging on national/ethnic/party purity or personality cults or idealizing moral progress have similar roots today.
Marxism was blatantly postmillennial Christian eschatology with the nouns swapped out, and a grand purpose presented for the faithful. Ayn-Randian fantasy is the satanist-equivalent inversion of this religion, uncritically accepting its flawed framework of the way the world works while inverting the value judgements in a way that very much does not lead to anything more functional.
Singulatarianism is similarly a blatant rehash of Christian eschatology, doctrines of redemption form original sin, and afterlife mythology, with different currents within it with isomorphisms to different schools of thought within Christianity. It is of course a far more niche interest than any of the aforementioned civil religions. Softer versions of talking about the Grand Destiny of Humanity Among the Stars are also related and more common.
Eventually all civil religions fail as they are limited by the rigors of the physical world and fail to provide the transcendence contained within mundane history they implicitly promise in the absence of more explicitly theological religions, which have more long-term staying power.
Self-help books represent the next step in the evolution of our collective consciousness. It might sound like a stretch, but if we consider that religion was once how society told its story to itself, then the recent erosion of religion is creating a new kind of story.
Norm-gatherers
Believers often defend religion with this rhetorical question: Without religion, how will people know right from wrong? But the standard rebuttal, as trumpeted by prominent atheists such as Richard Dawkins and Christopher Hitchens, is that everybody already has an innate understanding of right and wrong. Hitchens said that religion gets its morality from humans and that every society has forbidden perjury, theft, murder, and rape.
But notice what transgressions he cites: perjury, theft, murder, and rape. These are heinous wrongs that are obvious even to children. Obviously, lying is bad. Obviously, hurting others is bad. So to knock down religion for reiterating obvious morals comes across as straw-manning.
Another tactic that atheists use is to mock low-level norms, such as the Mormon practice of wearing Temple garments or the Orthodox Jewish ritual of Kapparot where you grab a live chicken by the shoulder blades and move it around your head three times.
Religious norms, therefore, receive attacks on two fronts: high-level norms, such as "Don't steal," "Don't lie," etc., have been rightly deemed redundant by modern, civil society; and, low-level norms, such as dietary restrictions, seem redundant thanks to modern inventions, such as refrigerators.
But there is still this vast middle zone of norms, which may be the most crucial normative output of religion. For example, respect for the elderly is a norm that has to be taught. Look at any teenager, and you can tell they "innately" don't care for old people. Or consider adultery. In some cultures, the practice is taboo, in others, it's a cause for being executed, and still, in others, it's just something not talked about in polite conversation, but definitely practiced in private. What should be the rule be about adultery? The answer isn't self-evident.
Example: dating and relationship norms
The way we establish norms today is a simulation of how norms were established in the past. Instead of Sunday mass, we have the podcast. Instead of the priest, we have the therapist. And instead of one holy book, we have self-help books.
Self-help books, just like holy books once were, are probably the most tangible artifact representing our society's norms. If you visit any bookstore, you'll see that the largest section of nonfiction is self-help.
You might be scratching your head at comparisons between the Bible and self-help books, but you can take any aspect of modern society and anchor it to ideas first popularized in self-help. For example, modern dating and relationships are heavily mediated by self-help. Books like The Five Love Languages or any book about the Myers-Briggs personality test are the lens by which single people view dating today. Case-in-point: on popular dating apps like Hinge and Coffee Meets Bagel, two of the most common prompts are: "What is your love language?" and "What is your personality type?"
Once you begin a relationship, you may shift to viewing it through the lens of books like Men Are from Mars, Women Are from Venus or Codependent No More. I remember my mom bought a copy of Men Are from Mars and then dog-eared and highlighted passages for my dad to read. In a different era, she would have taken her husband to church and squeezed his hand and nodded in his direction during parts of the sermon that applied to their marriage.
Even if you don't read self-help books, we all take cues from observing our peers, many of whom have filtered ideas from those same books. So, for better or worse, the self-help bookshelf is now a physical manifestation of our norms.
From Holy Book to Holy Wiki
It's not just norm dissemination that has been taken over by self-help. Self-help is now being used to answer deep questions that were once the mainstay of religion, questions such as, What should we do with our minds, and what should we do with our lives? For this generation, the answers are found in books like The Alchemist, The Secret, and Man's Search for Meaning, which are some of the best-selling books of all-time. One of these books or a book like them may out-sell the Bible by the end of the century.
So, while I started this essay with a specific, but cute, example of how self-help books have molded modern dating norms, it was meant to illustrate a broader point, that the decentralized bookshelf of ideas has disrupted the old technology of centralized holy books.
Society is now being molded by our individual choices, much like a wiki. Every time you pull a self-help book from that bookshelf, you implicitly increment its attention in the global soup of ideas. And just as Wikipedia is now a visible manifestation of our evolution in collective consciousness, so too is self-help.
(Cross-posted from Philosophistry)