Browse the wonderful world of Subject Headings. It's a bit like a thesaurus, only better!
We also use subjects on Open Library to gather together books that you can read in some form now, like:
Accessible book - Over 1,000,000 free eBooks, available to all in a variety of formats
Protected DAISY - Over 250,000 modern works, available for the U.S. print-disabled community
Lending library - About 10,000 eBooks available to anyone to borrow, 1 copy at a time, for 2 weeks
What's a subject heading?
As the wise Wikipedia says: "LC Subject Headings are an integral part of bibliographic control, which is the function by which libraries collect, organize and disseminate documents. LCSHs are applied to every item within a library’s collection, and facilitate a user’s access to items in the catalogue that pertain to similar subject matter."
Side note: Interestingly, professional library catalogers are often restricted to a limit of 3 in the number of subjects they may use. We collect general subjects, places mentioned, people mentioned, and even the time period the book is about. Feel free to imitate or enhance existing headings, or create an entirely new one. Or two. Or three. Or five.