In recent times, I've warmed up to a variety of themes from history, philosophy, economics and technology. Here's a list books that have shaped me. These are books meant to be less technical in flavor and rather more about putting broader perspectives in context.
See Textbooks on Various Subjects for books with more technical treatment.
LibraryThing is extremely helpful for managing book collection, and has a large & healthy community.
I like these resources that provide books that act as portals to domains that are usually complicated by jargon, or generally inaccessible.
Very Short Introductions is a great place for short and light reads of usually less than 200 pages. It covers a broad array of topics. I only use this resource to get a sense of fields that are alien to me.
Beginner's Guides is a collection of about 120 books (at the time of this writing) about the biggest ideas of our time. I have not sampled any book yet, but seems like a very rich collection!
Economist Books is potentially a good resource, given the quality of Economist "newspaper" itself.
GatesNotes maintains a popular collection of book reviews.
Blas Moros maintains an excellent library on The Rabbit Hole as well.
I am also curious about Alex Kasman's reading list on Mathematical Fiction. The recommendations are grouped by age/expertise; some titles look fascinating!
I also like the mission of Open Library, and ended up donating a tiny amount. I highly recommend checking them out as well, since they allow book loans.
Shepherd is an upcoming service that aims to bring the experience of browsing a bookstore to the web. It also has curated recommendations from authors. Hacker News Books newsletter is also often resourceful.
A Book Like Foo is an interesting search engine for books.