Hi Alfred -
It's almost certainly the cost of translating this hefty book along with all of the photos, table, colored maps and profiles in color that make this book so expensive. Helion's website shows 200 - 224 titles and nearly all of those that are hefty, required translating and have numerous photos, maps, charts, tables, etc., are in this same price range.
http://www.helion.co.uk/
AIR BATTLES OVER THE BALTIC 1941. THE AIR WAR ON 22 JUNE 1941 - THE BATTLE FOR STALIN'S BALTIC REGION
This title has not yet been published. Pre-order today, your card will not be charged until the book ships
Helion & Company 15th May 2018 Hardback 245mm x 170mm
528 pages 500 photos, 28 tables, 14 colour maps, 111 colour aircraft profiles