This is, in my opinion, the best book to get started into web application security both as a developer and as an attacker.
Although, the book speaks from a defender's perspective and focuses on how to improve your web application rather than hacking it, it can, however, also help you in your web application hacking journey.
The write up is absolutely brilliant with the best analogies to get you the core concept. If you're a beginner looking for a conceptual book then this is the ultimate book for you!