In what year do you think the popularity Star Trek peaked?
I don't think there can be much doubt that it was the early to mid-90s. People with no previous interest in ST or SF were watching and talking about TNG, and reinventing fandom. The online world had grown dramatically, and Star Trek hotspots like Compuserve and usenet were being joined by increasing numbers of websites. Magazines like Entertainment Weekly and TV Guide were doing special issues devoted to it. Industry magazines like Broadcasting & Cable looked at TNG as an important pioneer in first run syndicated drama.
And as for books... in 1996, there were nearly 80 Star Trek books, the most ever. Lawrence Krauss's 1995 book The Physics of Star Trek kicked off a whole new publishing niche that's still generating books today (all those The Science of... books). Major publishers were producing lots of nonfiction books about Star Trek. Pocket was publishing its main lines of novels and two series of YA books. St. Martin's Press published a seven book series of Star Wreck parodies. Small publishing companies were surviving almost exclusively on their unauthorized Star Trek nonfiction books.
Hell, the TNG finale was aired on the Jumbotron at Toronto's major league baseball stadium.
As much as I love DS9, I never had anyone offline to discuss it with until I met my wife. TNG? Pretty much everyone I knew watched it at least occasionally. And though TPTB tried to recapture some of TNG in Voyager and Enterprise, by that time the people I know who'd watched TNG regularly were barely aware that there was still any Star Trek on TV. They never got into DS9 (or Babylon 5, for that matter) or any other space opera-style SF series. TNG was a one-off.