Basically the Jewels of Phnom Penh is a list of 20 historic Phnom Penh buildings. The information included is pretty basic: year constructed, purpose and address, but the reason I liked flicking through was that for each picture they have a vintage photograph and a current photograph.
I think it would make a good starting point for a little walking tour of Phnom Penh.
I haven't done the full tour yet, but here are some shots of one of the 'Jewels of Phnom Penh', the mostly disused (although maybe not for too much longer) and recently restored, Phnom Penh Railway Station.