Hacking: The Art of Exploitation, 2nd Edition
This is one of the best books which will take you through the technicalities of areas like programming, shell code and exploitation. Regardless of whether you are a beginner or have very little hacking knowledge, this book will help you understand the complexities of the digital security tasks.
This excellent and well written book will make you learn all the clever stuff of getting access to a system. All in all, the best book to buy.

BackTrack 5 Wireless Penetration Testing Beginner’s Guide
Right from the beginning, this book gives you what you need, without wasting time in unnecessary justifications. Instead of explaining only theoretical concepts, the book consists of finely tuned and crystal clear tutorials. It provides a good mix of basics and high level knowledge and works cohesively with the reader.

CEH Certified Ethical Hacker All-in-One Exam Guide
This is undoubtedly one of the most well written books of all times. It provides crisp and clear writing with relevant examples along with a humorous touch to enliven the dry and mundane subject. The contents of the book are well organized in a neither too chatty nor too dry manner. However, you require some basic networking background to derive full benefits from this book.