Sitecore Helix Documentation version 2 in ePub format
If you don't know about Helix yet, but you have been working with Sitecore, you should have a look. And for others who have been enjoying the better world brought by Helix, Helix 2.0 was out in case you haven't heard.
And here is a copy of it in ePub format so you can download it and use your fav book reading app to learn all of it in style (Comparing to using browsers...):
This version is exact copy of release v2.0.0.