Zheng Yangwen's The Social Life of Opium In China is probably one of the best books out there on opium in China, concentrating mainly on how the substance was perceived by users and non-users. It's probably the most balanced and general treatment on the topic, though with of course a more social/cultural focus.

Opium Regimes: China, Britain and Japan, 1839-1950, edited by Timothy Brook and Bob Tadashi Wakabayashi, takes a more political-economic look, discussing the use and reception of opium in various contexts across its stated timeframe. Being an edited volume you can just go for whichever chapters suit your purpose and/or interest you.

A very hesitant recommendation, mainly for historiographical purposes, is Frank Dikötter, Zhou Xun and Lars Laamann's Narcotic Culture: A History of Drugs in China, which is for good reason rather controversial, as it can read as basically a pro-opium polemic intended as a sideways swipe at the War on Drugs, rather than a treatment of opium in its own context.

Stephen Platt's Imperial Twilight isn't principally about opium, but covers the origins of the Opium War at length.

For primary sources, Alan Baumler's Modern China and Opium: A Reader contains a good variety to choose from.