On behalf of Kelvin Lawrence, I'd like to share with the community this new resource that he has made available:

https://github.com/krlawrence/graph

For offline downloads:

http://kelvinlawrence.net/book/Gremlin-Graph-Guide.html

http://kelvinlawrence.net/book/Gremlin-Graph-Guide.pdf

As he mentions, This book is a work in progress. Feedback (ideally via issue) is very much encouraged and welcomed!