This was in my opinion the best and quickest source on good testing practice, which often meant change the code being tested:
https://misko.hevery.com/code-reviewers-guide/
Sadly, it looks like he has neglected his site, but the content can still be seen through google cache.
This page still seems to work and contains a much meatier amount of content: http://misko.hevery.com/attachments/Guide-Writing%20Testable...
I think Brendan Gregg's books[0][1] are quite inspiring for an understanding of a system with its inner workings, and provide information about various tools one can use to dissect a system and get some actual output in an automated way.
[0]: https://www.brendangregg.com/systems-performance-2nd-edition...
[1]: https://www.brendangregg.com/bpf-performance-tools-book.html