Can you recommend good resources (article/pdf, lynda, etc.) on writing tests? Less about the how (though important too) - but more about when, why, and for what

Can you recommend good resources (article/pdf, lynda, etc.) on writing tests? Less about the how (though important too) - but more about when, why, and for what parts/functionality. I find when I do write tests some are so trivial and then I stop doing them altogether.