These books help you learn about the discovery that happens at the beginning of a design project. Note that usability testing and similar methods are covered in the Usability evaluation section.
Just Enough Research
The title of this book is absolutely accurate. Erika doesn't waste any pages talking about obscure research methods you'll never use. Read this book, and you'll learn just enough about research to do what you need to on any project. A very easy and practical read, written by an entertaining author.
Universal Methods of Design: 125 Ways to Research Complex Problems, Develop Innovate Ideas, and Design Effective Solutions
This book doesn't just have research methods in it, but they make up a large part of it. It's safe to say that you'll probably never use all of the research methods listed, but if you need to learn more about specific techniques, or wonder how you should tackle a particular research problem, this is a great start.
Think Like a UX Researcher: How to Observe Users, Influence Design, and Shape Business Strategy