What books can I read to understand WW2 in China and why is there apparently so little work on the subject?

Mitter's Forgotten Ally is a good introduction to the subject. To this I would add Stephen R. MacKinnon's Wuhan, 1938; though it's short and specialized, the book effectively covers the Battle of Wuhan from a Chinese point of view. Shanghai 1937 isn't bad either, but (if I remember correctly) the author relies heavily on secondary sources.

I highly recommend The Battle for China if you can get your hands on it. The essays cover not only the course of the war but also narrower topics (e.g., the bombing of Chongqing, Japanese combat morale). You can also get a feel for the biases/perspectives of modern Chinese and Japanese historians. It's actually the main source for the sections on the Second Sino-Japanese War in Antony Beevor's The Second World War. Another great work of synthesis is S. C. M. Paine's The Wars for Asia, 1911-1949; this one's probably the most "bang for the buck," so to speak.

Diana Lary's The Chinese People at War provides an accessible social history of the conflict for the Chinese side. Highly recommended as well.

Tuchman's Stilwell is outdated and rather uncritical in repeating Joseph Stilwell's judgments about Chiang Kai-shek and the Nationalist government. Recent scholarship has done much to "rehabilitate" their image; good works include Hans van de Ven's War and Nationalism in China and Jay Taylor's The Generalissimo. You may also want to look at Jonathan Fenby's Chiang Kai-shek, though it's not as good as Taylor's biography, in my opinion. All of them challenge the notion that the Nationalists were hopelessly inept and that they avoided fighting the Japanese in order to conserve their strength for the Communists.

For Japanese perspectives, I would look at Edward Drea's Japan's Imperial Army (he also contributes a chapter to Battle for China) and Herbert Bix's Hirohito (though he obviously doesn't like the Emperor). The former offers an excellent overview of the institutional history of the Japanese army; the latter goes into some detail about the inner workings of the Japanese government and discusses their knowledge or perhaps endorsement of what we would consider war crimes.

Older, outdated books include Meirion and Susie Harries' Soldiers of the Sun (popularizing take on the Japanese army), Dick Wilson's When Tigers Fight (based mainly on eyewitness accounts of Americans and Europeans in China; Wilson is surprisingly sympathetic to the Chinese and harshly criticizes Stilwell), and James C. Hsiung and Steven L. Levine's (eds.) China's Bitter Victory (a collection of scholarly essays). Wilson's is especially interesting historiographically.

The closest thing available in English as an "official history" is Hsu Long-hsuen and Chang Ming-kai's History of the Sino-Japanese War (1937-1945); in fact, it's an abridged translation of the Nationalist government's official histories. It's filled with minute detail but is also essentially propaganda. Frank Dorn's The Sino-Japanese War, 1937-41 draws heavily from it, as does van de Ven's War and Nationalism in China. Personally, I would avoid Dorn's book unless you're curious about the historiography (he was an American military observer in China); it's very similar to Tuchman's Stilwell in attitude, somewhat Orientalizing, and very unfair in its evaluation of the Nationalist effort.

For the Japanese "official" perspective, there are the Japanese Monographs in English. These were written after the war by former Japanese officers (with American supervision), but many were subsequently updated or revised. They are not nearly as detailed as Hsu and Chang's History of the Sino-Japanese War.

I recently picked up Werner Gruhl's Imperial Japan's World War Two and would tentatively recommend it. While the author is not a professional historian, he's consulted a broad range of secondary sources and offers detailed estimates for the number of deaths across Asia during the Second World War.

Unfortunately, I cannot explain the relative lack of public interest in the Second Sino-Japanese War. Scholars, however, are still producing a lot of research on the topic. Indeed, our understanding the conflict has changed significantly over the past few decades; the books which I've recommended generally reflect the most recent trends. I hope you find this helpful! :)