Lean UX by Jeff Gothelf & Josh Seiden 
How to most effectively incorporate UX into your product process.

Two of the teams I’ve been part of over the years have followed this model. I wish it had been all of them.

Just Enough Research by Erika Hall
Quick guide to a few most-loved user research methods

Validating Product Ideas Through Lean User Research by Tomer Sharon 
How-to and when-to-use guide on several of the most common UX research methodologies

Universal Methods of Design by Bella Martin & Bruce Hanington
Overview of 100 ways to research humans problems, develop ideas, & design solutions

Design of Everyday Things by Don Norman 
Fundamentals of designing for humans

Don't Make Me Think by Steve Krug 
Helpful and funny primer on how to think about UX research

Product Design course by Udacity
Introduction to the product design process with a section dedicated to user research