This is purely a semantic discussion; whether you want to call it a "philosophy book" simply depends on what you define to be philosophy book. It is in the same boat as questions like:
Is everyone considered a philosopher?
How to earn the title of "Philosopher"
Can I introduce myself as a self taught philosopher?
You can argue all day and night about what percent a book has to contain "philosophy discussions" in order to be a philosophy book (is 30% philosophy 70% history discussion okay?), or how many peer reviews it has to have gone through (is 1 peer review the same as 5?), or the quality of those peer reviews (can some no-name philosophy magazine peer review or does it have to be the Philosophical Review?). These determinations are made on an institutional or personal basis and are essentially arbitrary.
Bottom line: It doesn't matter whether it's technically a philosophy book or not, what matters is what the book says...