There is also a "novel
" entitled "Planet X."
It's a Star Trek: The Next Generation/X-Men crossover.
No, I'm not kidding.
Written by Michael Jan Friedman and published in 1998. Apparently it's a sequel to something else, because in this novel the X-Men are already familiar with the crew of the Enterprise. They're pulled from their time, and their universe, seemingly at random, when their timehook goes ka-blooey. While hanging out on the Enterprise, they get the chance to be cosmic do-gooders and help a planet whose inhabitants are beginning to develop mutant powers of their own.
At the risk of editorializing, the book really isn't very good (IMO anyway). It's got the subtle backstory of the X-Men, with the heavy-handed preachiness we've all come to expect from Star Trek. Plus the whole concept is just plain silly.
There are worse ways to waste a couple hours than poring over its 265 pages, but I'm having a bit of trouble thinking what they might be right now.