How to learn Scala cheaply and freely

Dateline July 15, 2021:

The book is about 250 pages long, and I could easily charge $25 for it. But I want to make them available at as low a cost as possible to make it as inexpensive to learn Scala as possible.

Note that this book was released in October, 2018, and covers Scala 2. If you want a free version of a similar book, the online version of the Scala 3 Book is completely free.