When describing topics related to unit tests, I try to support my knowledge through professional literature, reading books, articles or watching video materials. Below is a list of materials that I particularly like and would like to recommend.
The newest and greatest book on unit testing in my opinion. Contains theoretical knowledge with practical examples. Here you will find answer for the question why unit tests are important, what code to test, and what is a good candidate to mock. A very good source to consolidate your knowledge.
Another good book focused on Test Driven-Development, which is advanced approach of writing tests. I like mainly for describing examples from which you can learn TDD very easily and intuitively.
One of the first books that describe an approach to writing tests. It describes very well the code smells that can appear while writing tests. Today we can find many more books on this subject, but if you want to find out where it all began, this is the book for you.
If you know any good source of knowledge and what to share with me then feel free to contact!