In the Star Trek: The Next Generation books, how can Geordi La Forge be in a relationship with Tamala Harstad? I thought he was in a committed relationship with long-term girlfriend / Future wife Dr. Leah Brahms.
He's not up to no good, is he?
Let's start with the most important tidbit of information:
The stories you read in the novels are not considered "this is how things are in the Star Trek universe." They're great stories, so by all means enjoy them. However, the various shows and movies don't depend on them in anyway, and different novels need not refer to each other either. If one book has LaForge in a relationship, a different one could show him to be single, married, having a slew of one-night stands, or whatever. Different novels can be independent of each other. This answer is from the canon perspective.
Now, the premise of the question is that Geordi LaForge has a relationship with Leah Brahms, so the book must be depicting LaForge as cheating on Brahms. This is not so.
We know about Leah Brahms through two epsidoes: Booby Trap and Galaxy's Child. In Booby Trap, LaForge creates a holographic simulation that features a life-like image of Brahms in order to help work through the engineering issues the ship was having during the episode. The computer ended up programming Brahms to be pretty friendly, at which point LaForge develops an attraction to the simulated Brahms.
About a year later, Leah Brahms visits the Enterprise-D in the episode Galaxy's Child. About half way through the episode, LaForge discovers that Brahms is married. At the conclusion of the episode, they have become friends. This makes it clear that LaForge is not in a relationship with Brahms.
GEORDI: Looks like you've got leaf miners. Might want to use a bacillus spray. (off his look) My wife is quite a gardener. I've picked up a little bit of it.
Geordi takes a tie and begins carefully tying the vines. Picard bends down with a little difficulty... and the two of them start to work on the vines.
PICARD: How is Leah?
GEORDI: Busy as ever. She's just been made director of the Daystrom Institute.
The reference to Geordi's wife in this timeline appears to be Leah Brahms. Levar Burton confirms this in a panel at 2016's Salt Lake Comic Con FanX.
Question: The Leah referenced in “All Good Things…”, was that Leah Brahms?
However, this is moot since this timeline is destroyed when Picard, with Q's help, solves the temporal anomalies and returns the space-time continuum back to normal.
Books aren't canon, so discrepancies can (and do) happen.