F. Kohl, E. Rosler Anatomy of the Ship The Type XXI U-Boat,  30 bucks, thi is a new book with dj in mylar with a tiny spot on the inside cover--see photo-- so reduced price. Fritz Kohl served on uboats and Rossler is the historian , a great combination. Everything you want to know about this uboat is here, scale drawings of everything so you can accurately build any plastic model or build from scratch.. lots of photos here and likely on the web.