not to dredge up this old discussion, but Avatar to Unity is season 8, Worlds of DS9 to The Soul Key is season 9.
Which is just one opinion, not a universally accepted rule. I still think that if you're going to impose a "season" structure on the novels (which you really shouldn't with any seriousness, because they were specifically designed not
to mimic television story structure), it's a better fit to treat Unity
(set in September) as the big February sweeps event and Olympus Descending
(which ends on December 31) as the cliffhanger season finale. Warpath, Fearful Symmetry
, and The Soul Key
were meant to be the beginning of the next phase after that, but that phase was cut short due to Marco's layoff and the decision to jump the series forward in time.