This is a succinct - and entertaining - look at the mortgage collapse. While the title suggests otherwise, it doesn't just explain the sub-prime mess, it explains the logic behind the entire mess.
I picked this up from this Michael White's site, where the discussion centers around solving the real estate crisis by paying down all the mortgages in America by 20%. It's going to cost about $5 trillion, but the discussion has merits. You can take a look at the discussion here.