The best single online source to explore the Cold War in general is the Wilson Center’s Cold War International History Project. And for a helpful chronology of lips and teeth during the Cold War, with links to primary source documents, check out the Wilson Center page on China-North Korea Relations.
For a snapshot of PRC-DPRK relations, along with other useful links, the Council on Foreign Relations’ Backgrounder on the China-North Korea Relationship is a great resource . Another useful treatment can be found in Dick K. Nanto and Mark E. Manyin, China-North Korea Relations.
And, for a pair of deeply thoughtful research articles on the evolution of the relationship, you can study its beginnings in Shen Zhihua, “Alliance of ‘Tooth and Lips’ or Marriage of Convenience? The Origins and Development of the Sino‐North Korean Alliance, 1946‐1958,” US Korea Institute Working Papers (December 2008), and then reflect on how history plays into the present by reading Heonik Kwon, “The Korean War and Sino–North Korean Friendship”, The Asia-Pacific Journal, August 12, 2013.
© John Delury