Depends on which series you're interested in. It also depends on whether you want something that's a good read or something that follows on from a series.
Two good TNG related books are The Buried Age (about Picard's early years) and Death in Winter (picking up after the events of Nemesis. For DS9, I might suggest A Stitch in Time, which tells the tale of Garak after he leaves the station and is written by the actor who portrayed Garak. Voyager has Mosaic and Pathways as two books telling the backstory of some of the characters, as written by one of the producers.