Hi Tim. My first thought when people talk about testing is test-driven development, but there is more to it than that. Is TDD what you've got in mind?
I'll go with TDD, as I've got a suggestion — Kent Beck's Test-Driven Development By Example. The last person I recommended it to said "this is the most enjoyable programming book I've read. Kent is hilarious!"
People often talk about two different approaches to TDD. They're sometimes referred to as "London" and "Detroit", after the groups that popularised each approach. Kent is in the Detroit camp, and consequently many XP'ers are too (myself included).
For a good overview of the differences, I like Martin Fowler's article:
https://www.martinfowler.com/articles/mocksArentStubs.html#SoShouldIBeAClassicistOrAMockist
I linked to the end, but the whole article is quite good. If it's a bit tl;dr (it is long), you could stash it until mock objects come up...