I didn't think I'd ever see a book (and it looks like a nice one) about this obscure aircraft.
A little known fact is that the Douglas B-18 Bolo was America's most numerous front line bomber at the time of Pearl Harbor.
The full title is "Douglas B-18 Bolo - The Ultimate Look: from Drawing Board to U-Boat Hunter" by William Wolf (ISBN 978-0-7643-2581-6) USD $59.95 (retail).