Here my input.
1. Allyn Wallace
This man passed away earlier this year but even today he is still an inspiration to many. His book is so good and _for me_ a must-have for any astro photographer.
A video was released last week to promote his book in a special edition:
I also advice to check his various videos on YouTube
2. Peter Zelinka
Very good courses (bought a few) and also organiser of workshops (never done one)
Peter Zelinka is a nomadic nature photographer and instructor who specializes in astrophotography.
www.peterzelinka.com
3. Lamce Keimig
And maybe you find this gentlemen his course something interesting:
https://www.creativelive.com/class/astro-photography-lance-kreimig
Hope this can help.