[Jtsummers]

Working Effectively with Legacy Code by Michael Feathers. It spends a lot of time on how to introduce testability into existing software systems that were not designed for testing.

Property-Based Testing with PropEr, Erlang, and Elixir by Fred Hebert. While a book about a particular tool (PropEr) and pair of languages (Erlang and Elixir), it's a solid introduction to property-based testing. The techniques described transfer well to other PBT systems and other languages.

Test-Driven Development by Kent Beck.

https://www.fuzzingbook.org/ by Zeller et al. and https://www.debuggingbook.org/ by Andreas Zeller. The latter is technically about debugging, but it has some specific techniques that you can incorporate into how you test software. Like Delta Debugging, also described in a paper by Zeller et al. https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=988....

I'm not sure of other books I can recommend, the rest I know is from learning on the job or studying specific tooling and techniques.

[GrumpyYoungMan]

TDD is a development methodology, not a testing methodology. The main thing it does is check whether the developer implemented what they thought they should be implementing, which is not necessarily what the spec actually says to implement or what the end user expects.

[Jtsummers]

It's still a useful technique and way to apply testing to development. But yes, it's not the best resource in telling you what tests to write, more about how they can be applied effectively. Which is a skill that seems absent in many professionals.