I really recommend "The Real North Korea" by Andrei Lankov. He's a professor of Korean history who actually got to study at Kim Il Sung university as an exchange student, and knows the country as well as any non-North Korean alive.
But keep something in mind. If China were controlling North Korea, it would be much more sensible to establish their own base with their own nukes (or, at most, just give the North the nukes but keep all the proprietary information to themselves) than allow them to develop an independent program.
North Korea has been much more of a necessity for the Chinese than an ally or a puppet. If they were the latter, they should have followed the same economic reform path China's been treading for nearly 40 years now, instead of the world's last Stalinist state and the only communist nation to have a hereditary line of succession (although we'll see what happens in Cuba when Raoul finally kicks it).