Case in point: YOU didn't buy the tech manual either.Counterpoint. There are people that buy tech books even if they aren’t interested in the literature or even the property it’s based on. I have my fathers TNG tech manual, and the only other Trek book he owned was the old ‘Making Of’ from the sixties...
If the 15 people in my family who have extensive collections of books on their shelves I can think of only two who actually possess a copy of the TNG tech manual: my mom (who got it at a Star Trek convention way back in the day) and my dad, who got it as a birthday present from me after my mom kept all the Star Trek stuff in the divorce. I also gave him my copy of Mister Scott's Guide to the Enterprise.
So that's three sales out of a family of fifteen EXTREMELY avid consumers of books. And that's not even to say the rest of my family doesn't have literally DOZENS of Star Trek books on their shelves too, they just don't give a shit about the technical aspects because they aren't engineers and aren't pretending to be. My cousin actually dropped $30 on a book of Star Trek sheet music and plays the song from "The Inner Light" on her piano, and my youngest uncle bought two copies of the Star Trek Chronology and gave me one.
ART books do, yes. Technical manuals, not so much.They sell, and not only to Trek fans. Model makers, design students, general genre fans, gamers of various stripes, engineering students...all sorts.