1. Contextual design, holzblat How to understand and structure a user's life and environment and what to look for when they are struggling with a problem or using your product.
2. Lean Customer Development, Cindy Alvarez Practical and immediately useful guide to interviewing and talking to users. Read after the Contextual Inquiry book above. If that book teaches what to understand about a user's workflow and environment, this one teaches how to elicit those answers from customers when talking to them.
3. About Face 3, alan cooper If the previous two are about (a) what to look for when understanding a customer and (b) how to get the info you need to build solutions for those customers, then this book is about translating those learnings into user journeys and experiences.