Mastering Bitcoin - by Andreas M. Antonopoulos (opens in new tab)

This has been submitted to HN multiple times, the most popular discussion with 45 comments is [1]. One new thing is that the book is moved from Antonopoulos' personal Github account into a Github organization specific to the book

[1] https://news.ycombinator.com/item?id=7493916

[2] https://github.com/aantonop

Along similar lines:

https://MasteringMonero.com

Available as a Free PDF, paid eBook, or a physical book for your bookshelf. Discounted/Free paperbacks also available.