I mean, this is sort of a broad question -- are you looking for a summary of the period? Or just cool facts that stand out?

Either way, one of the best books I read in college was Soldiers Alive by Tatsuzo Ishikawa. It was written in the late 1930s, during the Japanese invasion of China, and follows a platoon of soldiers as they press deeper into Chinese territory. It's (quite frankly) a disturbing book about war and what war does to the people who fight it, but it's so worth the read.