Ethical Hacking is the best ethical hacking book for anyone who wants to learn about the basics of ethical hacking. Hilary Morrison and Hein Smith do a great job at covering the different types of attacks that can be carried out against computer systems, how to find vulnerabilities in systems, and how to write exploits that can take advantage of these vulnerabilities.
This book is also a great resource for understanding the psychology behind social engineering and how to evasion intrusion detection systems.