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.
resource-recommendation questions are on-topic but they need to follow the guidelines in our resource recommendation policy. Before asking a resource recommendation question, do your utmost to check whether there is a request about the same topic and whether the resources there are sufficient. Answers to resource recommendation questions must not be link-only or they will be deleted; we expect answers to describe the resources they recommend above and beyond what one might learn, for example, by trawling through Amazon.
Contrast this with the specific-reference tag, which is for questions that ask for a specific resource, instead of any resource explaining a particular topic. The tags resource-recommendations and specific-reference are mutually exclusive; they should never both be applied to the same question.
The resource-recommendations tag also covers journals and non-paper electronic materials, such as, e.g., video lectures, on-line resources, etc.