It's not so much about being an "expert" as being able to explain technical things in a way that most can understand. Before I start writing a book, I spend several weeks with the camera, shooting action, nature, and events, and testing it with Nikon accessories. I shoot several thousand pictures with the camera in a relatively short time. I read the manual from front to back seeking new bits of information buried deeply in obscure locations. I compare my experience with the new camera and the older Nikons.
Then I start writing. As I come to each individual function, I test it thoroughly, comparing it to previous Nikons and noting any new features. I go through each menu while configuring the camera and taking more pictures to make sure I understand how the function works. Then I write it down easy-to-understand language.
Bottom line: None of the individual functions in a Nikon camera are overly complex. However, there are hundreds of functions that can be configured in multiple ways. The manual is hard for most people to deal with because it puts bits of important knowledge in multiple places, instead of just one place. My books pull all related functionality together into one place, explain things in common everyday terms, and add my personal experiences and recommendations for each setting.
Anyone could do exactly what I do, by spending a lot of time with the camera and figuring out how it works. The reason people like my books is that I try my best to make it easier for them to learn about their camera. I am just a hillbilly from Tennessee with a flair for explaining things. Given enough time anyone could ferret out the details by themselves, and many do; however, it is a waste of time when someone has already done it for you. My books are for Nikonians who would rather have a little help instead of digging through a manual. I did it for you, that's all!