For non-fiction books my approach is markedly different and I seek out a lot of online and peer reviews. With non-fiction books being a more significant investment (time and cost vs expected advancement of knowledge) I can spend weeks deciding which books will be useful to me.
I also recommend Farnam Street Blog (http://www.farnamstreetblog.com/) for a lot of interesting suggestions.
I still find a lot of success in just going to a second hand book store and finding gems in a fairly short amount of time too. But obviously more niche books are unlikely to be there.
Goodreads, does tend to be fairly good at identifying authors of a similar theme, but that has only been of limited usage to me.
For each subject in which I am interested, I maintain a bibliography. The most common way entries are added here is by discovering references to books or articles in other things I am reading.
When reading books in physical media, I usually draw an arrow by references or notes that indicate books I should add to these bibliographies. When I am done with a book or article, I go back through and add any such references that still seem relevant to my bibliography. (I do this at the same time I am transcribing notes and quotations, files of which are also an important part of my system.) I usually include a brief note on why I believe this work will be of value and where I got the reference.
Where this becomes useful is when I am pursuing some sort of study. I have learned that I study best with some object in view, so even if my study is for general education, I prepare a syllabus that presents objectives of some sort (code to write, questions to answer, etc.).
Once I have the objectives prepared, I use my living bibliography files to create a reading plan. Of course, once I really delve into a subject, that plan usually expands and I add more references into both my general bibliography and reading plan.
This process provides me an apparatus to pursue my interests in a structured way to make the most of my time. I don't worry much about what to read next except in the context of some such study.
I have found that without this sort of structure to focus my reading, I am in danger of becoming a dilettante. I don't have an exceptional memory, and if I read without a structure, I easily fall into the trap of cultivating superficial knowledge without ever getting deep enough to answer substantive questions or do original work.
I do, however, want to encourage serendipity, so I subscribe to and read periodicals in variety of fields. I have also found The London Review of Books and book reviews in leading newspapers to be useful sources of references and lines of thought that I would probably never otherwise discover.
When it comes to technology, I find books to be a secondary citizen. If I look at things I know and use in my day job. Things like stack overflow, blog posts and reading man pages have been my primary knowledge sources.
But off late, I've been reading books and mostly enjoying the titles I've read. But the biggest problem for me is finding my next book.
Amazon mostly recommends titles very closely related to what I've purchased, and I haven't had much success with goodreads lists. So I'm curious how HN users find their next book.
1) New releases from publishers/editors/authors I like. If you like 3 books from one publisher, you're probably more likely to like another book from that publisher despite its topic. Same for editors. And, for authors although author releases are less frequent than editors which are less frequent than publishers.
2) Colleagues and friends. Word of mouth still seems to be the number one way people discover books. Ask your friends what they are currently reading. Hop over to GoodReads.com and see if you can find a network that reads genres similar to what you like.
3) Blogs/forums. We like to talk about what we're reading since that makes an otherwise rather solo activity more social. Search the personal/academic/company blogs for topics you like.
Finally, why not create a hacker news thread called "What are you currently reading?" :)