1. Thom Hogan - Complete Guide to the Nikon Z9
also see his depth Z9 review - https://www.zsystemu...era-review.html
2. Jason Odell's latest eBook, Bird Photography with the Nikon Z9, is now available.
3. Steve Perry. Secrets To The Nikon Autofocus System: Mirrorless Edition. This updated edition adds much detail to lengthen an already detailed ebook. free download to owners of previous editions
I prefer the deeper more comprehensive explanations by Steve Perry and Thom Hogan. Both these authors drill into the core functions of the camera.
To date, there are already at least 3 fairly detailed ebooks available on the Z9 - all published 3-4 months after the first cameras trickled out from Nikon.