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