Christopher's Ex Machina, a TOS book set after The Motion Picture, might also be a fun read, but his writing style is pretty dense, so it might be too much for you if you prefer the shorter ones.
Q Squared is an excellent call; I'd actually recommend pretty much anything by Peter David before he wrote New Frontier, with other highlights being Vendetta (read after The Best Of Both Worlds) and Imzadi (long, but worth it). Diane Duane's Dark Mirror is also a lot of fun, even though flatly contradicted by more recent Mirror Universe continuity.
Good standalone DS9 novels are pretty uncommon; in addition to the two mentioned so far, I think for my money the only other one that's much worthwhile is The Siege, also by Peter David (I may be something of a fan of his).
Ex Machina - tough but brilliant
Vendetta, Imzadi, Q-In-Law, Q-Squared - Peter David's TNG books
Dark Mirror, by Diane Duane, for an older look at a possible mirror universe
The Siege, Fallen Heroes, The 34th Rule - for my money, the only really good DS9 standalones.
That's good for starters.