August 2020 Update
Whilst I still recommend those "classics" below I would have to add to the list:
Chaos Engineering System Resiliency in Practice
Continuous Delivery: Reliable Software Releases through Build, Test, and Deployment Automation
May 2011 Answer
The art of Software Testing is a time-honoured classic that is very good.
The testing chapters of Code Complete by Steve McConnell are excellent.
Chapter 5 of Microsoft Secrets is really good for understanding how the daily build works. (But his is getting hard to find now)
How we test software at Microsoft is also a good title.
