Indeed there isn't! But there's still some. See this thread and this thread, both with recommendations by /u/hellcatfighter and /u/DrDickles. A couple of books that aren't on there but also useful to have a look at are Kwong Chi Man's War and Geopolitics in Interwar Manchuria: Zhang Zuolin and the Fengtian Clique During the Northern Expedition, whose title basically explains itself; and also Edmund Fung's The Military Dimension of the Chinese Revolution: The New Army and its role in the Revolution of 1911, which concerns the formation and development of the late Qing army that would later morph into the core Warlord forces under the Republic.