there is so many books ...before???

Hi, I cannot understand why in this sentence it is used there is and not there are; besides I cannot understand the structure of the sentence considered as a whole
There are many books that I can read about a serial killer (????)
before (???) I'm bored.

A woman says:
There is only so many books that I can read about a serial killer before I'm just bored and have had enough.

I understand the overall meaning: there are many thrillers which bore her; but I cannot understand how the sentence is constructed. Please help me understand how the fdifferent parts of the sentence are linked to each other!!!! Rip.

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.