AFAIK, Quinn won't be around anymore, but Choi definitely is. Alvaro in the Crossroads will also be gone but you'll meet another merchant in the area just after the Crossroads, after that you'll be in enemy territory so you won't get too much help, but by then you should be used to destroying the English Army.
I don't think the book quest will be completable at the point you're at (you can check your Active Quests to be sure though).