If you always dreamt of a book about Active Record, you may have a serious disorder needing treatment, but if you just wished that you had something that you could reference, I have something for you... Allow me to present the Active Record API Documentation Book in its full glory, over 400 pages of detailed documentation, usually cover a lot more than you need to know.
This is a result of running DoxyGen over the ActiveRecord project, it gathered all the XML comments and produce a lovely PDF. A while ago I spent a significant amount of time ensuring that each and every method in Active Record has at least somewhat meaningful XML comment. So I feel more than a little pride in it (at one point I was looking at close to 700 compiler errors, IIRC).
This is not a book about teaching how to use Active Record, it is the API reference documentation, nothing more.
You can get it here.