I estimate that about 3/4 of the photos in The Camera and the Tsars, by Charlotte Zeepvat, are formal. However, I just had a *quick* look through it, and found only about half a dozen of OTMAA from the 1914 sittings, and just one from 1913. The book focuses on the entire Romanov family from the mid 1850's on, so if you're most interested in OTMA, it may not be the best book for you.