I recently started reading online about a few of the more “colourful” warlords of 1920s China (Zhang Zongchang, Zhang Zuolin) but I’m not sure which books are the best to cover this era. Does anyone have any good suggestions?
For Zhang Zuolin and the Fengtian Clique you might be interested in:
Kwong, Chi Man. War and Geopolitics in Interwar Manchuria: Zhang Zuolin and the Fengtian Clique During the Northern Expedition. Brill, 2017.
Suleski, Ronald Stanley. Civil government in warlord China: tradition, modernization and Manchuria. Peter Lang Pub Incorporated, 2002.
McCormack, Gavan. Chang Tso-lin in northeast China, 1911-1928: China, Japan, and the Manchurian idea. Stanford University Press, 1977.
Shai, Aron. Zhang Xueliang: The General Who Never Fought. Springer, 2012.
I don't think there's anything in particular that addresses Zhang Zongchang's role outside of the books mentioned above, but given his prominent role in the Northern Expedition, you might find some details here:
Jordan, Donald A. The Northern Expedition: China's National Revolution of 1926–1928. University of Hawaii Press, 2019.
Chi, Hsi-sheng. Warlord politics in China, 1916-1928. Stanford University Press, 1976.