from what I have - in order of goodness
Odekirk, Glenn E. Spruce Goose (Title inside cover: HK-1 Hercules: A Pictorial History of the Fantastic Hughes Flying Boat). Long Beach, CA: Glenn E. Odekirk and Frank Alcantr, Inc., 1982. No ISBN
McDonald, John J. Howard Hughes and the Spruce Goose. Blue Ridge Summit, PA: Tab Books Inc., 1981. ISBN 0-8306-2320-5 or
McDonald, John J. Howard Hughes and his Hercules. Shrewsbury, UK: Airlife, 1981
Howard Hughes: An Airman, His Aircraft, and His Great Flight (Hardcover)
Thomas Wildenberg (Author), R. E. G. Davies (Author) - last one you can easily skip