The answer will depend on your interest in Wu and your language skills, but in all cases the basic problem remains the same; the sources we have are very restricted and for the most part treat the empress as an example for future rulers to learn from, rather than as someone whose thoughts and deeds and character should be set down and analysed as objectively as possible. Moreover, for someone as unique and as well-remembered as Wu is, there's a surprising dearth of detailed biographical studies.

The most recent popular biography in English is Jonathan Clements' Wu: the Chinese Empress Who Schemed, Seduced and Murdered Her Way to Become A Living God (2007), which, as the title indicates, is not averse to retelling some of the more scurrilous gossip about its subject. Two more scholarly studies that place Wu in her context are Richard Guisso, Wu Tse-T’ien and the Politics of Legitimation in T’ang China (1978) and Dora Shu-Fang Dien, Empress Wu Zetian in Fiction and in History: Female Defiance in Confucian China (2003).

I wrote about Wu and her reputation in an essay for the Smithsonian which can be accessed via my blog site here. This essay offers a brief intro to the main problems of writing about the empress and to the historiography of the reign, if that's of any interest.

Thank you for your comprehensive reply. I will certainly explore those options. I’m looking at the essay now, mst enjoyable. Is her tomb really intact. “... like many of the tombs of the most celebrated Chinese rulers, including that of the First Emperor himself, it has never been plundered or opened by archaeologists.” How tantalising that is!