I'm sure this will be moved to TrekLit, but I chime in...
If you want to fill in some blanks and then move on to the newest adventures, here's my recommendations:
TNG
Filling in some blanks:
Ship of the Line (The first voyage of the Enterprise-E)
The Battle of Betazed (Enterprise-E during the Dominion War)
Diplomatic Implausibility (An Ambassador Worf novel)
A Time to Be Born
A Time to Die A Time to Sow
A Time to Harvest
A Time to Love
A Time to Hate
A Time to Kill
A Time to Heal
A Time for War, A Time for Peace (A nine book series following the Enterprise-E in the year before Nemesis explaining all the changes that took place)
Articles of the Federation (A political sequel of sorts to ATFW, ATFP)
Taking the Enterprise-E crew beyond Star Trek: Nemesis:
Death in Winter (A Picard adventure)
Resistance (The Borg return)
Q&A (The ultimate Q novel)
Before Dishonor (Continuing from Resistance)
Greater Than the Sum (Continuing from Before Dishonor)
The adventures of Riker and Troi on the USS Titan:
Titan: Taking Wing
Titan: The Red King
Titan: Orion’s Hounds
Titan: Sword of Damocles
And some of the most popular that I didn't include above:
Vendetta (Borg vs. Planet Killers)
Imzadi (The ultimate Riker/Troi novel)
Kahless (The full story)
Triangle: Imzadi II (The whole Worf/Troi storyline)
The Sky's the Limit (An era spanning anthology)
DS9
Some works of note:
Terok Nor: Day of the Vipers
Terok Nor: Night of the Wolves
Terok Nor: Dawn of the Eagles (A trilogy spanning the entire history of the Ocupation)
Prophecy and Change (A series spanning anthology)
The Lives of Dax (An anthology avout each host)
A Stitch in Time (The ultimate Garak novel)
The Relaunch (set after the series finale)
The Left Hand of Destiny 1
The Left Hand of Destiny 2 (Ambassador Worf's first adventure)
Twist of Faith (Omnibus edition of the first five Relaunch entries)
These Haunted Seas (The next two books)
Mission- Gamma 3: Cathedral
Mission- Gamma 4: Lesser Evil
Rising Son
Unity (The culmination of the first part of the relaunch)
Worlds of Deep Space Nine 1: Cardassia and Andor
Worlds of Deep Space Nine 2: Trill and Bajor
Worlds of Deep Space Nine 3: The Dominion and Ferenginar
Warpath
Mirror Universe: Fearful Symmetry
The Soul Key
The Never-Ending Sacrifice
VOY
Of note:
Mosaic (Showrunner Jeri Taylor's Janeway biography)
Pathways (Showrunner Jeri Taylor's biography of everyone else)
Distant Shores (Series spanning anthology)
String Theory: Cohesion
String Theory: Fusion
String Theory: Evolution (Trilogy explaining the many little unanswered questions and continuity flubbs threwout the series in an epic way)
The Relaunch (set after the series finale)
Homecoming
The Farther Shore
Spirit Walk 1: Old Wounds
Spirit Walk 2: Enemy of My Enemy
Destiny
An era spanning crossover trilogy that will completely change the future of TNG, Titan, DS9, and Voyager:
Destiny 1: Gods of Night
Destiny 2: Mere Mortals
Destiny 3: Lost Souls
And the aftermath:
Losing the Peace (Post-Destiny TNG)
Over a Torrent Sea (Post-Destiny Titan)
Full Circle (Pre- and post-Destiny Voyager)
A Singular Destiny (The political fall-out)
All that should keep you busy for years.
