Complex sentence: The book was republished in 2010 by Skyhorse Publishing, with a new introduction by historian Roger Moorhouse, who indicates that "his (Dietrich) insights are sound and sincere, but the obvious question which arises is: when did they occur to him?".
Split into simpler sentences: The book was republished in 2010 by Skyhorse Publishing, with a new introduction by historian Roger Moorhouse. Moorhouse indicates that "his (Dietrich) insights are sound and sincere, but the obvious question which arises is: when did they occur to him?".