Is it considered haram for the pages of a book you borrowed to be affected by some folds or light saliva spray during studying?

What you mentioned about using the book in a way that does not cause damage or harm—there is no sin or harm in it. The objective is to preserve and protect the book, and to be careful not to expose it to damage or intentionally spit on it. However, if the book is damaged by what was mentioned or by other means, then there is a detailed ruling regarding the guarantee of borrowed items.