Surprisingly, because it's not my usual reading genre, it recently was "John Wayne Gacy, Defending a Monster" by Judge Sam L. Amirante and Danny Broderick. I noticed it on the recently returned list at my library while browsing and it got my interest when I read how he got his attorney.
I was in the middle of another book but I started Gacy and just couldn't put it down. It's possible it was just a much creepier read for me being familiar with the area and knowing the exact locations, buildings, and even the bridge he dumped bodies from. But I checked it out one day and returned it 2 days later.
And I'd have to agree the feeling was like maxbookworm said... it was like watching a train wreck you couldn't take your eyes away from.