Riga at War 1914-1919: War and wartime experience in a multi-ethnic metropolis (Herder-Institut, 2014) by Mark Hatlie can be ordered through any German bookstore and through the usual online sources:
The American and British Amazon pages offer it as well, but it can be ordered for a lower price from amazon.de. You save money ordering from Germany despite the cost of shipping.
Contact / Impressum:
Dr. Mark R. Hatlie (ViSdM)
Im Feuerhägle 1
info @ hatlie.de