Usually, I buy military books used from amazon.com (still cheaper to pay the postage from USA than ordering from amazon.de).
In addition, I keep looking for the books I am interested in on Kindle, because they usually are cheaper in electronic format.
The last ebook I purchased was:
Seapower: A Guide for the Twenty-First Century (Cass Series: Naval Policy and History) by Geoffrey Tiller. The price was 28,99 € (versus 41,99 € for the paperback and 121,99 € for the hardcover.)
I have not read it yet, but will let you know, if it's worth the purchase. :)
P. S. two other ebooks which I recently bought for the Kindle are
"Dreadnought: Britain, Germany and the Coming of the Great War" and "Castles of Steel: Britain, Germany and the Winning of the Great War at Sea", both written by Robert K. Massie. Price was around 6 € (!) each...