Project Gutenber free ebook :
Submarine U-93
http://www.gutenberg.org/ebooks/39387
The Submarine Hunters: A Story of the Naval Patrol Work in the Great War
http://www.gutenberg.org/ebooks/26641
A Sub. of the R.N.R.: A Story of the Great War by Percy F. Westerman
http://www.gutenberg.org/ebooks/50578
A Sub and a Submarine: The Story of H.M. Submarine R19 in the Great War
http://www.gutenberg.org/ebooks/47745