So I was just wondering, if a book is like 4mil would it increase the intelligence by more than one? Or would it only give it one?

Nope, all books give one intel point.

Aside from stat boosters, that is.

So then what is the point of buying really expensive books?

Well, once you've read all the lower ones, that's all that's left. And stat boosters run just as, if not more, expensive.

Ohh ok thank you very much for your help :)