Every once in a while, we get a question asking for a book or other educational reference on a particular topic at a particular level. This is a meta-question that collects all those links together. If you're looking for book recommendations, this is probably the place to start.
All the questions linked below, as well as others which deal with more specialized books, can be found under the tag resource-recommendations (formerly books).
If you have a question to add, please edit it in. However, make sure of a few things first: