A Stitch In Time
is Garak's autobiography, written by the actor who played him. It's fantastic.
Articles of the Federation
is sort of Star Trek meets The West Wing, if that seems interesting. It's a little bit tied into the ongoing continuity of the post-series novels, while still standing well on its own.
is one of my favorite older books. It's been years since I read it, so I can't say how well it would hold up now, but it's just a lot of fun.
I'll also second the mention of Imzadi