Below are books that I have found most useful while pentesting. The downside to offensive security is that things change rapidly and books can quickly become out-of-date. There are still frequently books coming out that are worth buying, but for reading I spend most of my time looking at high quality blog posts. I will still callout books I like here, but it isn’t going to be the most up-to-date reference for the best current books on hacking, so do some research on your area of interest for the best books currently available.
The Hacker Playbook 3 – Finally a new version of THP is out and it is good. This version is more focused on Red Teaming than version 2, but it adds some up to date techniques and knowledge.
Hash Crack: Password Cracking Manual – This book is the same small size as the RTFM and is an excellent reference for Hashcat and other cracking tools.
RTFM – Great reference for command line syntax, networking and etc. It is small and easy to carry with you.
The Hacker Playbook 2 – While a couple years old, it actually provides some good techniques and references that are still very useful.
Wireless Hacking Exposed, Third Edition – Practical details on wireless assessment of WiFi and other wireless networks. It is a great reference if you only do wireless testing from time to time.