There is only so many books that I can read about a serial killer before I'm just bored and have had enough.
ONLY SO MANY is a phrase in this case. It means that there is only a handful of books that she can actually read before she has had enough (an unspecified but small amount). In other words, once you've read one...you've read them all: they follow the same pattern, the same gruesome crime scene, the same perpetrators, same victims just different methods of killing, etc. That's why her interest is quickly gone.