I agree with this, I find it much easier and probably more fun to browse a subject at the top level, but I also experience a sort of pleasure in obtaining a complex formalization of an idea that wraps it up in a neat package. I think that it is for this reason that I can pleasurably read a mathematics text in say, Model Theory or Set Theory or Lambda Calculus (to a somewhat lesser degree).
I do notice that my pleasure in learning a subject increases with respect to two variables: the apparent breadth of it's implications and the applicability it has to my general understanding of the world. Together these seem to amount to how much stuff I can conceptually organize in to a formal framework with the information.
Maybe if you have an over arching goal in mind it could help you out. My broadest goal is to be able to consider as many objects and events as possible in formal frameworks so that I have a clear picture of how things interrelate. I immensely enjoy being able to break down what I might be mulling over or what I have experienced in novel ways to try to get insight in to the subject. So, when I research something related to history, it is because I am worried about modern politics, possible misinformation and about why I might take a certain stance on an issue rather than an obsessive desire to pick up neat little nuggets of information.
I have found that cycling through a specific spectrum of ebooks gives me a good result. What I do is I read a text on a subject I find interesting until it becomes difficult to follow, and then I switch to learning something else, and break it up with diversions whether it's watching a video of some sort or playing a game of Go etc. I generally find that when I come back to the subject (I don't have a good time frame for this) I have a much more intuitive grasp of the material I had previously learned, and can often anticipate certain things that appear in the new material. I then can easily go further along in the book than I could before. This allows me to maintain my level of fun while going deeper in to a subject without burning out. A caveat: I enjoy working problems, so if you don't (depending on the subject), you probably will hit a ceiling as far as learning and maintaining your fun level goes.
On the other hand, you might just find yourself meandering ebooks (if you have a collection as large as mine and you are like me, you will). However; I have generally found that I meandering through technical ebooks can often lead to new insights and new areas to explore and can often lead to a better understanding of previously explored subjects. It is like wikicrawling on steroids.
I am realizing something that many, many intelligent people are guilty of - collecting and hoarding and accumulating crap, useless information. This is dangerous, because it feels like you're doing something useful, but you're not.
However, speaking personally - once I decide to start focusing and researching something systematically to get better at it, it gets harder to do. For instance, I taught myself statistics mostly using baseball stats. It was a fun, easy, harmless context to learn statistics.
I read lots of history and historical fiction. I read up lots on business and entrepreneurship. This is easy and fun and enjoyable.
But then, when I decide to really hone in, it becomes much harder. For instance, I'm doing some casual research on the history of insurgencies and asymmetrical warfare. This is the kind of thing I'd read all the time for fun, but now that I'm working on it systematically, it becomes a lot harder.
Likewise business and entrepreneurship - I read lots and lots on technology, financing, market research, marketing, etc. But now that I'm really nailing down one aspect for my next business, it becomes almost strenuous to work on that.
It's like... collecting and hoarding useless, unfocused information is for us what collecting and hoarding a bunch of useless consumer shit is for most people. I'd reckon that people that hang out here are smarter with money and less into buying junk, but, at least for me, I'm spending a lot of my time buying junk information.
Alright, back to reading about Tienanmen Square and Rome/Carthage and the Tet Offensive, and nailing down the buying criteria and budgets of the market I want to be in. Why it is so much easier to focus and collect crap mentally than to do it systematically on meaningful topics? Do you do this? I seriously doubt I'm the only one...