@AhmadKhaled you didnt indicate your bkg. if you already are familiar with basics of big-O notation, try something more advanced. this is an internet draft of a book that is well regarded/ widely used, Computational Complexity, a Modern Approach, Arora/ Barak theory.cs.princeton.edu/complexity/book.pdf

@AhmadKhaled I don't like Sipser much, I found it disappointingly shallow more than once. Ullman's " Introduction to Automata Theory, Languages, and Computation" is a good classic for automata and computability. I don't know a good one for complexity, personally.

Not sure if our books questions are of any help.

@AhmadKhaled Sipser is widely used also. havent heard complaints like Raphaels. if you want to learn it, get every book you can get your hands on. theres a lot of stuff available on the net now also wrt videos etc eg open courseware, coursera etc... another strategy is to look for courses that cover what you want to know (syllabuses) & find out what books/ content they use. agree with Raphael that Hopcroft/ Ullman is excellent but definitely quite dated at this pt. (1979)