"KiCad Like a Pro, 3rd Edition" written by Dr. Peter Dalmaris from Tech Explorations is great reference for KiCad users of all skill levels. This book covers everything from getting started using KiCad to using git for project version control. A special edition of this book has been created of which half of the profits from each sale will be donated to the CERN & Society Foundation for KiCad development.
Several text-based tutorials as well as video tutorials have been put together by various KiCad users. Here you can find a limited subset.