I have been searching for the title of a TOS-TNG crossover novel for some time. I believe it was published in the early 1990s, as I do not remember Voyager or DS9 being on TV. It was also before ST:Generations was made. I know it is not the novel Star Trek: Federation, as the two crews actually work together and meet one another. Also, I seem to remember that the front cover said something to the effect of, "at last, the generations meet!"
How about Star Trek: Crossover?
"At last, the generations meet" appears on the cover of the book, Unification
This is a novelization of the two-part TNG episode with the same name. I think that the only TOS character who appears, though, is Spock.
It was released in 1991.
Published in 1994. I bought it at a Waldenbooks in a mall near Groton, CT when I was 18. I was right out of boot camp 😆