Cem Kaner/James Bach/Bret Pettichord - Lessons Learned in Software Testing
If you were only allowed to buy one single book about software testing, then this would be it. A very good read and the structure is partitioned in individual lessons.
Harry Collins - Tacit & Explicit Knowledge
It might surprise you but this book will actually teach you why testing certifications do not work. You want to know why? Read yourself. And on your way you learn a lot about learning, too.
Lee Copeland - A Practitioner's Guide to Software Test Design
Now, this is really valuable for the ones of you that are new to software testing. Very solid. And Lee is such a nice person. A must read for the newbie to become more proficient.
Jonathan Baron - Thinking and Deciding
Thinking is a good thing. And knowing what errors may happen during thinking is even better. If you are somebody who really wants to know how things work then this is for you.
Tom DeMarco/Timothy Lister - Peopleware: Productive Projects and Teams
This is for the the book for the managers among you. Don't make the mistake of micromanaging or thinking that you need to be "on top of things". It's people with feelings that make your team - not machines.
Mihaly Csikszentmihalyi - Flow: The Psychology of Optimal Experience
Well, I am not too sure if this really needs to be recommended as the state of flow is already almost common knowledge. Anyway, this is the original an well worth a read.