My go-to for learning resources and references would be the Awesome List, and one of them is about testing: github.com/TheJambo/awesome-testing. Although I would say I'm not strong on the fundamentals of testing itself, more to the practical usage of it, so I can't comment much on this. If anyone has any books to recommend, let both of us know!
P.S.: If you're a frontend developer, this video is one of my favorites on how to test your components (the context is Vue, but there are points in the video that are applicable to any other frontend framework).
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