It depends on how many cases you have.
If "A and B" are two things which will both occur together, and you want to describe that single scenario, then you would write "case," because there's just one case, the case of both A and B. This is especially true if "A and B" form a fixed phrase such as gin and tonic.
If "A and B" are two things which might occur separately, but the book provides information in both of those scenarios, then you would write "cases," because you have two. You might imagine that we should use "A or B" in this construction, but that is not required, because it's an abbreviation of "the case of A and the case of B."
When using "the case of A or B," the singular is acceptable (and, arguably, preferred), because we again have a single case, in which either A or B is present. You could in principle write "the case of A or the case of B," but that's a bit harder to follow, and probably not a great idea to abbreviate.