Few weeks ago I found a book about GitLab authored by Jonathan M. Hethey.
It includes next chapters:
Book is easy to read and gives basic information about GitLab application.
This book is a good option for people who are not familiar with GitLab. For developers - it explains application functionality and describe possible workflow. For administrators - how to setup, update application or where to look for help.
Here is a link to the website where you can buy a copy - packtpub.com
Want to know more about book? Its a good review from Axilleas Pipinellis (contributor to GitLab)