Thom Hogan is very highly regarded by Nikon users, as pRON says.
Personally I don't care for eBooks so much -- I find a physical reference book
much more useful. That being said, I've bought two of Thom's eBooks (for the
Fuji S2 and the Nikon D200) and have been glad I have them on hand.
I consult the Nikon SLR forum on dpreview.com far more than any reference
book when I'm trying to figure something out about my D200. Just about
anything I've been confused about or needed to know has been discussed
to death there, accessible with a search or two.
.................................................................................................
*We are just a quarantined people under an evil sun.*
--- Richard S. Shaver