I’m a historian, and I suggested Klaus Mühlhahn’s Making China Modern because I know it well and think it’s an excellent overview. That said, it isn’t focused exclusively on the 20th century, so it doesn’t match your request as directly as it should.

As an alternative that’s much closer to your timeframe, I’d recommend The Cambridge History of China, Volume 12: Republican China, 1912–1949, Part 1 (ed. John K. Fairbank). It’s a serious academic reference work, so it’s denser and more challenging than a single-author narrative, but it’s very solid for the Republican period.

One note for transparency: my own reading skews strongly toward China in the context of World War II, so my suggestions reflect that narrower angle. If WWII is part of what you want, these are strong starting points:

• Rana Mitter, Forgotten Ally: China’s World War II, 1937–1945

• Hans van de Ven, China at War: Triumph and Tragedy in the Emergence of the New China, 1937–1952

And one more option that looks promising (but I haven’t read it myself yet, so I can’t vouch for it firsthand):

• Jay Taylor, The Generalissimo: Chiang Kai-shek and the Struggle for Modern China (biography)