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