Hi David,
Perhaps "The Complete Blue Max: A Chronological Record of the holders of the Pour le Mérite, 1740-1918" by Kevin Brazier?
I read the one and only review on amazon.co.uk which states that the book deals mainly with airmen and no other branch, which is a little disappointing, though I believe some sort of a list of all recipients is attempted.
Another is "German Knights of the Air 1914-1918" by Treadwell and Wood.
This book deals solely with the airmen winners but I believe it gives biographical information as well as photos.
Are there any books with information on all Pour le Mérite holders of WW1?
Kind regards
Pierce