Best compilations by a mile for Real Disco (slept on stuff, mostly produced after the mainstream thought disco was dead) are Disco Spectrum from BBE. Compiled by the great Dave Lee aka Joey Negro. Three volumes I think. The first one in particular is soooo good.
I have a thing for westcoast label Solar, which featured artists like Shalamar, Dynasty, Carrie Lucas, The Whispers, and other decadent outfits from late 70s/early 80s. Sequel Records issued a bunch of Solar compilations back in 1999 - Solar Disco Classics is what you need.
Ten years ago I would've told you to get anything you can find by Larry Levan, but nowadays there's tons of compilations and mixes of his best stuff out there, and some of them are quite poor. I would check out Journey Into Paradise: The Larry Levan Story and Live at the Paradise Garage - good summaries of the man's DJ skills, his remixes and his eclectic musical sensibilities.
The always reliable Mastercuts guys have a couple of great compilations too. Check out Classic Salsoul Mastercuts, vols 1-2, Classic 80s Groove Mastercuts, and Classic Disco Mastercuts (natch).
EDIT: Yes, as Gibby says, get the hits by Black Ivory and the man behind it all - Leroy Burgess. There are two Burgess anthologies out from Soul Brother Records called Anthology vol 1: The Voice and Anthology vol 2: The Producer, both of which are great and well worth checking out.
That should get you started.
