I, Q by John de Lancie and Peter David. The universe is being swallowed up by a huge whirlpool dubbed “The Maelstrom” and de Lancie’s Q actually tries to save everyone.
Planet X by Michael Jan Friedman. I’m a huge X-Men fan as well as a Star Trek fan and this book is a crossover. I actually really enjoyed it and if I remember correctly it actually references another cross over that happened in the comics. There’s even a bit where they poke fun at the resemblance between Picard and Professor X; the book came out before the X-Men movies were cast.
I think these books weren’t well reviewed, but I loved them.