The DS9 numbered novel Fallen Heroes
is a great read, as is The 34th Rule
. Both are early-seasons stories, although I don't recall exactly when they're supposed to fall. Doesn't matter too much in either case.
A Stitch In Time
is the definitive Garak novel. If you've only seen DS9 season 1 you've only encountered him once (?) so far, so you may want to hold off on that for a bit, but put it on your list.
Truly standout TNG novels seem to be fairly uncommon, but Q-Squared
is certainly one of them. Good pick. Q-in-Law
is pretty good as well.