Goodman Games ran a kickstarter last year for a setting book and adventures based on Fritz Leiber's stories. Although the print release has been delayed, the pdfs were released to backers a few months ago and I finally got the chance to look over them a little. The city guide for Lankhmar in particular interested me as a useful tool for fleshing out Khromarium.
The city guide gives a brief history of Lankhmar. The city has stood for over a millennia, with the current inhabitants having settled around the ruins of ancient towers and stone temples. They paid homage to the inscrutable gods depicted there and the city has been able to flourish. In Hyperborea, those ruins were the old city of the Hyperboreans and the temples introduced the barbarian races to the worship of Xathoqqua.
After the history is a description of the government and guilds of the city - only a handful of the nearly 100 trade guilds are named, leaving plenty of room for development. There is a section on Crime and Punishment which uses DCC's Luck attribute - I'd change this to a Charisma check.
After this it gets into descriptions of the eleven districts of the city. Each is given a short description, a few notable landmarks, and an Interesting Events table to spur some adventure seeds. This is all system neutral and would only need swapping out of some religion and culture names to fit into Hyperborea.
There is a 6 page folio of important characters from Fritz Leiber's stories statted up for DCC which is the least useful section for an AS&SH game, but it's a pretty small portion of the book.
Lastly, there are lots of tables for creating neighborhoods, streets, and buildings in the city which should be useful in any large city setting.
I've been reading over Rats in the Walls and Other Perils and Lankhmar complements it perfectly to fill in the docks around the Silvery Eel or the neighborhood of that dilapidated building of the Brazen Bull, and any entanglements with the Thieves' Guild that may occur from stealing the Lamia's Heart will benefit from Lankhmar's infamous Thieves' House.