I once skimmed "How to win friends and influence people". I didn't read enough to have a good opinion of the advice (I suspect djcb's description of it being fairly good advice as long as the author's experience generalises well, which HTWFAIP probably does better than many but not perfectly).
However, what had a profound influence on me was that though there's an unfortunate stereotype of people who've read too much Carnegie seeming slimy and fake, the author seemed to genuinely want to like people and be nice to them, which I thought was lovely.
In the very back of Kaj's excellent How to Run a Successful Less Wrong Meetup Group booklet, he has a recommended reading section, including the classic book How to Win Friends and Influence People.
It just so happens that not only have I read the book myself, but I have written up a concise summary of the core advice here. Kaj suggested that I post this on the discussion section because others might find it useful, so here you go!
I suspect that more people are willing to read a summary of a book from the 1930s than an actual book from the 1930s. What I will say about reading the long-form text is that it can be more useful for internalizing these concepts and giving examples of them. It is far too easy to abstractly know what you need to do, much harder to actually take action on those beliefs...