Questions asking "What is a good book about X?" or more generally, "What should I read to learn about X?", typically where there is not a single authoritative reference. These sorts of questions have multiple answers and will acquire new answers as more books/papers are written on the subject. Note that such questions are Not Constructive if they do not specify a narrow topic and level of understanding.
Contrast this with the reference-request tag, which is for questions that ask for a specific resource, instead of any resource explaining a particular topic. The tags books and reference-request are mutually exclusive; they should never both be applied to the same question.
The books tag also covers journals and non-paper electronic materials, such as, e.g., video lectures, on-line resources, etc.
Please add your question to the book list as well.