Below are the books that I have found most useful while pentesting.
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.