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. GatesNotes maintains a popular collection of book reviews as well. A Book Like Foo is an interesting search engine for books.
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.
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!
I am also curious about Alex Kasman's reading list on Mathematical Fiction. The recommendations are grouped by age/expertise; some titles look fascinating!
Blas Moros maintains an excellent library on The Rabbit Hole.
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.
Here is the complete list of books reverse sorted by year, fetched from my account at LibraryThing. Often some titles may have an older publication date. I've provided my rating alongside each work.