Regarding the Amazon book, read the Accepted Answer in this post https://world.episerver.com/forum/developer-forum/-Episerver-75-CMS/Thread-Container/2017/9/your-episerver-the-missing-manual-book-feedback/
Regarding Jon's ebook, I've bought it (but haven't read it much yet) because I've referred to a lot of Jon's blog posts in the past and want to support his contribution to the Episerver community. The book is really cheap, although the website says it's 55% complete.
If you are working with Episerver Commerce I can strongly recommend these books by Quan Mai https://leanpub.com/u/vimvq1987
I don't really agree that there isn't enough Official Documentation https://world.episerver.com/documentation/developer-guides/ but I do agree it can be a lot to consume when you are starting out. Maybe you can try the official Training Courses https://education.episerver.com/
I advertised my book on pre-sale for a while, I think someone ripped off the name to try and make some easy cache. I'd avoid the amazon book as it's just a copy of the free documentation here.
My book is a work in progress, I've spent over 100 hours on it so far. I'm working in sprints so a new update gets pushed every two weeks. Up to you if you think it's worth the investment. The thing I would say is that I do update and add bits based on feedback, so far quite a few people have brought it and feedback which is amazing. I'm really not getting rich from this book, it is a guide to help people out
Darren, thanks for buying a copy.... what a legend!
Has anyone read this book?
Amazon has another by the same name, but seems to be a different author.
The episerver online documentation is pretty limited.