Technician Research Stage 3

I am at the Stage 3 Technician level of progress in TD2. In order to move on to Stage 4 I have to destroy the turret before killing Books.

When I last went in, I didn’t even see Books anywhere during the mission. Where does he show up?

Also do I have to play through all the invaded missions again to have it reset? Seems like I can’t get back in there to replay.