This is my second favorite mystery series (Edie Claire's "Never" series being my favorite). It is written by identical twins, about mirror twins. The chapters alternate between the twins, which some might find annoying, but I find fun.
It is second favorite because of its humor. The story lines are not particularly original (are any?), but it rolls along with lots of laughs and is comparable in plot to most mysteries I read.
The series includes: Double Murder, Double Exposure, Double Cross, Double Dealer, and Double Date. I did not read them in order. It would be better to do it that way though.