I recently got hold of a copy of Lennart Lindberg's "U 3503 - Dokumentation" (ISBN: 91-85944-30-0) detailing the escape, scuttling, salvage and scrapping of U 3503. The boat tried to reach Norway during the closing days of WWII but where forced to scuttle in Swedish waters after mechanical problems. It was later salvaged by the swedes and its secrets were absorbed into the Swedish submarine force.
The preparations in gremany and the escape from anti submarine aircraft and the sinking of U 534 which were in convoy with U 3503 and U 3017 is described briefly.
The book is a little thin regarding information on the more technical aspects, no mention of what types of torpedoes were carried, what "secret" technologies where captured etc. The only exception is the snorkel mast which was installed on a Swedish boat for trials and stood model for later versions.
What makes this book worthwhile is the 219 b&w photos of the crew and commissioning ceremony,the scuttling, the salvage, salvage fleet and divers, the cutting up and re-use of parts on swedish subs. I don't know how much reference is available elsewhere on the XXI type but photos of the underside of the keel has to be quite unique. The photos of the scrapping can make almost any one cry "why?".
A bonus is all text is available both in English and German as well as Swedish.