These Deep Space Nine tales took place before the events of Avatar.
The Lives of Dax
and Prophecy And Change
Edited by Marco Palmieri
The Lives of Dax takes a look at each of the Dax symbionts hosts
The stories in Prophecy and Change take place throughout the run of the show.
Double Helix book two: Vectors
by Dean Wesley Smith and Kristine Kathryn Rusch
Set during the Cardassian occupation, this book teams up Dr Kathryn Pulaski and Bajorn freedom fighter Kira Nerys to save Terok Nor- the future Deep Space Nine- from a deadly virus.
A series set on occupation era DS9, called Terok Nor, should be released 2006ish.
The 34th Rule
by Armin Shimmerman, David R George III and Eric Stillwell
Co-written by the actor that plays Quark. Referenced in Twikight by David R George III.
Hollow Men
by Una McCormack
Hollow Men is a follow-up to the DS9 episode In The Pale Moonlight.
The Left Hand Of Destiny books one and two
by J.G. Hertzler and Jeffrey Lang
Co-Written by the man who played Martok, The Left Hand Of Destiny tells the story of the newly enstated Chancellor of the Klingon High Council.
Diplomatic Implausibility
by Keith R A DeCandido
Although its a TNG novel, this book picks up Worfs story from the end of the DS9 television series and The Left Hand Of Destiny.
A Stitch In Time
by Andrew J Robinson
Garaks life story by the man who brought him to life.