The first edition of my Complete Guide to the Nikon Z6 II and Z7 II is now available for purchase. You'll find it in the books section this Web site. And yes, there are enough differences between the original cameras and the new ones to justify having a separate book. The last time I tried to cram more than two cameras into one book it became a mess of qualified statements and tables trying to point out differences. I prefer—and I think you will too—the clarity of simplicity and directness. I also suspect that the cameras will deviate some as Nikon updates firmware in the future, too.
My review of the Z6 II will probably come next week, as I'm still testing one aspect of the camera and the weather is not cooperating.
