The Better Book Question
How many copies have I sold? That's one way to assess your success. Asking another one often makes more sense.

Selling books en masse has always been hard. Don't let anyone tell you different.
Intellectually, that statement probably makes sense to you. If you're a first-time author or haven't penned a book in a while, though, it's unlikely that you know how hard it is. Give Helen Lewis' article a read if you want to know the particulars. TL;DR: Even many rock stars struggle today to get others to buy, read, and recommend their excellent texts. For every Andy Weir, there are thousands of scribes scraping by.
Regardless of their choice of publishing methods, rare are the nonfiction authors who don't ask themselves the following question: