A nice touch is the progress chart included in this activity book.
Fighting Elite: U-Boat Crews, 1914-45 by Gordon Williamson (1998, HardBack )
What I like about this activity book is how it encourages creative thinking.
U-Boat Crews 1914-45
A nice touch is the stickers that mark your progress in this activity book.
Osprey – Elite Series 60 U boat crews 1914-45 – Book
This novel weaves together multiple voices without ever feeling messy.