In recent times, I've warmed up to a variety of themes from history, philosophy, economics and technology. These are books meant to be less technical in flavor and rather more about putting broader perspectives in context.
📗 See my books library at LibraryThing.
LibraryThing is extremely helpful for managing book collection, and has a large & healthy community.
📚 Also see Textbooks on Various Subjects for books with more technical treatment.
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!
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!
Computing the Human Experience has books about the past and future of technology.
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.
Five Books provides (expert-)curated list of five books on a variety of topics. Shepherd is another 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.
Planet EBook has free e-books for many popular classic titles.