Blaze of Glory and The Romulan Prize are pretty good TNG novels that feature Romulans as the antagonists. Sadly, since the author only did three Trek novels, including those two, they get lost in the mix of the early Trek tie-ins, before the editors made everything into one continuity.
Michael Jan Friedman's Crossover was pretty good book too, especially since it seemed to be a more fitting story for Generations.
I don't know if this counts as "less known," but I've always felt the New Earth/Challenger series is pretty under the radar, despite the fact that it seems quite popular with libraries.
The Brave and The Bold was a nice little minseries that fleshed out some of the characters that had been mere guest stars in the show, plus it was my first introduction to the IKS Gorkon cast, which is another novel series that seems to be under the radar.