A mathematical proof:
1) Hypothetically, a GURPS book can be about anything.
2) If the topic is generalized to "anything," there can be no such thing as too many books.
3) Therefore, there can be no such thing as too many GURPS books.
4) GURPS books are a subset of RPG books.
5) Therefore, there can be no such thing as too many RPG books.
Quid erat demonstratus.