Hi there! This is Property-Based Testing with PropEr, Erlang, and Elixir, a book on Property-Based Testing. It's pretty much the friendliest intro to property testing you'll find.
The book teaches you how to automatically generate test cases, test stateful programs, and change how you design your software for more principled and reliable approaches. You will be able to better explore the problem space, validate the assumptions you make when coming up with program behavior, and expose unexpected weaknesses in your design. PropEr will even show you how to reproduce the bugs it found. With the book, you will be writing efficient property-based tests in no time.
This website is essentially an older draft copy of the book. It hasn't gone through editing, only contains Erlang (no Elixir), and is not as well-organized. It also does not contain the latest info on PropEr, such as targeted properties. All that stuff is only available if you decide to buy a copy of the real book.
In the meanwhile though, and because I want people to have easy access to this information, you can still visit this website as it was before the book got published. Do note that I am not keeping the website up to date however.