Descent — Diane Carey

The Borg are back -- and nastier than ever. No longer mindless, inhuman drones but cyborg soldiers on a mission to assimilate all intelligent life, their new goal is the complete destruction of the Federation. And in the midst of facing the Borg again, Captain Picard learns that he may be the cause of the change.
Commander Data is changing as well. For the first time in his life, Data feels a genuine emotion -- and that emotion is deadly, destructive rage. When it is revealed that the Borg are commanded by an old enemy out of Data's past, Data must choose between his allegiance tot he Federation and the one thing he wants above all else...