It may be difficult to write books for the expert, but I believe it's relatively easy to write for the expert.
When writing for the beginner or intermediate, pedagogy is as, or more, important than content. One has to arrange and explain to convey novel ideas. When writing for an expert, one can simply describe at a high level (eg non food-porn cookbooks, jazz fake books, etc.) with the confidence that the expert brings in enough outside context both to gloss over any repetition of what they do already know, and to figure out (after possibly closer study and external cross-reference) even compressed descriptions of what they don't already know.