What literature can be read about comparing different approaches to testing?

Question

Software testing

Good afternoon.
Tell me what literature (books, articles) or courses you can read (see) about comparing the main approaches to testing (possible in English), which discusses the advantages and disadvantages of each method?
PS: you need to test a ready-made .NET application written in C# / WPF. Now the project is in the optimization phase, so you need to cover the most basic functionality with tests to be sure that after the changes, the main functions will work as before. We thought about Unit tests, but I'm interested in what alternatives there are.
Thank you for your attention

Answer 1

Roman Savin. "testing DOT com" or A Handbook on Bug Abuse in Internet Startups
This book is more than enough for you. The main thing is to apply what is in it in practice.