Ask HN: How to improve my automated testing skills?

I understand the fundamentals of unit and integration testing. I try to use a balance of them in all code that I write. However, specially with languages such as python and JavaScript, I am constantly stressed about breaking the "integration" between functions at some boundary which isn't being tested in my test suite, even though the suite has a high coverage.

I have recently subscribed to Destroy All Software screen-casts where the speaker does demonstrate some similar scenarios and tries to resolve them. Are there some similar books/ videos/ open source projects which I can refer to, to improve my testing process and suites?