Lambda the Ultimate

Hello,
I was wondering if anyone knew of good explanations of HM-type inference works, with an emphasis on implementation. I've found some papers using citeseer, but all of them seem to assume that the reader has a PhD in computer science and go WAY over my head (I only program as a hobby). I did some google'ing and found The Essentials of Programming Languages, but if possible, I want to avoid having to pay ~$50 just to learn one thing. After reading SICP online and after learning Haskell, I decided to write a toy Lisp interpreter with type inference, but the implementation details for the type inference escape me. What I liked about SICP was the emphasis on implementation and how they provided source code for everything. Is there a similar (freely available?) PDF that explains type inference? I apologize in advance if I accidentally posted this to the wrong forum, or if this has already been answered many times. I perused the archive a little, but I didn't find any similar requests. Although I read LtU, this is the first time I've ever posted anything.
Kind regards,
Kevin