The facts of Kim Il Sung's life continue to be debated by scholars. Kim Il Sung by Dae-Sook Suh is a generally well-regarded history of Kim's life, but understand that it is far from complete and not necessarily accurate.

Kim's "autobiography" With the Century can be read in it's entirety online, but be warned: it is quite a lot of reading and is largely unverifiable myth and propaganda. I would like to add that while it is not a reliable source of factual information, it is a must-read to understand North Korea's own perception of Kim, and therefore not devoid of value.

As for North Korea's "early years," I recommend Politics and Leadership in North Korea by Adrian Buzo for a general overview, and Crisis in North Korea by Andrei Lankov for analysis of the August Faction Incident, which is considered a formative event in the development of modern North Korean politics. A Misunderstood Friendship by Zhihua Shen and Yafeng Xia is a favorite of mine that generally explores the evolution of Chinese-North Korean relations, but helps to fill what I consider some gaps in Western understanding of North Korea.