Vesa Sirén is his name and his book is called “Suomalaiset kapellimestarit, Sibeliuksesta Saloseen, Kajanuksesta Franckiin” (or, “Finnish Conductors: from Sibelius to Salonen, from Kajanus to Franck”).
As far as I can tell, there is no English-language edition yet available, though I’m sure it would be a bestseller in the U.S.
Sirén told his newspaper that he wrote the work “in the wee small hours of the night. As a rule, for two or three hours from the moment when I managed to get my child off to bed.” He also took a five month leave of absence, funded by the Finnish Cultural Foundation.
Among the tidbits in this 1,000 page tome: Sibelius had a good conducting technique “unless he was drunk.”