The go-to recommendation (which I am surprised you have not received yet) is Rana Mitter's Forgotten Ally: China's World War II. Mitter is a highly credentialed historian of China at Oxford.

I also think the relevant section in Search for Modern China, written by Yale's Sterling Professor Jonathan Spence, is also really good for how short it is.

A bit longer time frame than you asked for, but I’d highly recommend “The Wars for Asia 1911-1949” by S.C.M. Paine. It presents the Chinese Civil War, the Second Sino-Japanese War, and World War II as three nested conflicts, and does a great job providing context for the escalating Japanese invasion. You say you just read about the civil war starting from 1945, so you’d learn a lot from the decades leading up to that point.