TLDR: I thoroughly enjoyed “Beautiful Testing: Leading Professionals Reveal How They Improve Software” and would recommend it to anybody wanting to read about how tests are actually designed, written and maintained in the “real world.”
One of the best features of this book is that it is not language or culture specific, so you get to learn how other people and communities test their code, which often shows you a new way at looking at your own tests.
This book has an entire chapter about “fuzz testing” and a few different fuzz testing frameworks, that was extremely enlightening. Of all the kinds of testing that people do, fuzz testing is probably the least well known and hardest to find good introductory information about.
The book also contains a chapter by John D. Cook, who is a statistician by trade that also has the additional rare quality of being a great writer. Even though I consider myself quite knowledgeable in higher mathematics and statistics, I learned new ways to test these kinds of things. It should be considered recommended reading for anybody trying to test things in numeric algorithms, scientific computing and related fields.
I found the chapter about how Clam Anti-Virus does testing to be quite entertaining. They definitely have a unique and complicated testing environment that should make anybody feel better about how hard their tests are to write.
The “Testing One Million Web Pages” chapter by Tim Riley describes how Mozilla tests Firefox and is another exciting read about how to scale your test suite.