Wow, I have no idea what the answer to that might be. I liked TOS and perused the library copy of TOS Technical manual, made copies of the Enterprise floor plans, and puzzled over the logic of some of the other speculated starfleet ships that simple rearranged the exterior components into odd configurations. I know somebody who liked the Dreadnaught variant enough to cannibalize Enterprise model kits in order to cobble one together. Or maybe it was a Destroyer class. The appearance of the Reliant and Kelvin make those more likely than I originally thought, I suppose. Even so, to this day I still don't know what to make of them. I think it makes it less awkward when they don't take a nacelle that looks exactly the same as the Enterprise's and put it elswhere on the ship, but rather come up with a different looking nacelle. I think that's why I feel a certain curiousity and acceptance toward the depiction of the dreadnaught on the cover of Diane Carey's Dreadnaught! novel. I never really bothered to read the text of that tech manual, as I was more interested in the visuals.
I recently skimmed through some of the text of Mr. Scott's Guide to the Enterprise, which I liked better as TOS Movie's counterpart to TOS Technical manual (preferred the refit configuration of Enterprise). There's some interesting stuff in there about two companies developing different variations of a more advanced Warp engine, based on scientific data from The Tholian Web episode. I prefer Kirk's newer Enterprise being renamed from The Yorktown to MSGttE's explanation that it was the Ti-Ho, it's more symbolic having it renamed from Yorktown. The dating is so off, though, isn't it? A used copy of the book was recently procured for me by a family member, and it's very nice to have, but I wouldn't have actively searched it out myself.
I do remember the ST:TNG Technical manual being very popular around the time it came out, which was a peak moment for ST. TNG was popular and watch by many in school, without shame, and the sixth movie came out to much enthusiasm. Around this time I was in Junior High. I can't remember what the catalyst was for getting it, but I definitely caught the fever of, "I must own that soon!" My dad agreed to buy it for me, I think, and we sat down for lunch and my dad asked if he could look through it. And I waited eagerly, craning my neck hungrily, as he flipped through it for an hour or so, maybe. A good friend of mine, who started watching TNG because I talked about it so often, also looked through it extensively. And then went and got his own copy. Quite a number of other friends and casual aquantences had their own copies. We were mystified and fascinated by the whole thing about Warp 9 and 10, and what happens to object moving that fast. Peter David did it great with Vendetta. The Voyager episodes interpretation evokes contempt, nice way to kill the mystery.
I started to drift away from Star Trek around the middle of DS9, but I still looked in on it occasionally. I didn't feel to preoccupied with the notion of a technical manual for it. I have to say, I don't even know if the other shows got their own technical manual.
I'm afraid this response has gotten long winded, but I don't know the actual figures for how well sales were for the different technical manuals. I just know how my generation seized on TNG's tech manual as a window into understanding the technobabble that cropped up often enough in the show. It was neat to understand what they were talking about when they said structural integrity field or inertial dampeners, and technobabble was still a fun game for trying to guess at what was being done. It all got a bit much, though, didn't it? Other shows and movies have shown how satisfying it is to go light on that stuff. So while TNG was fun for all that stuff, Voyager and Enterprise were shocking because they were still going at it, and made me feel fatigue.
Too much rambling info...?