From the Steve Jackson Games web site:
Abydos: The merest whisper of the name sends shivers down the spines of the God-fearing people of Yrth. What lurks in the forbidden city of Abydos is so great a heresy, the Church suppresses all mention of its very existence. But such loathing has no meaning for the shambling, silent bodies who roam the streets of Abydos. The undead are the greatest resource of the necromantic priests who practice their dark heresies in that city. Their beliefs and mandates are backed by the blades of highly trained soldiers and a very silent majority.
Written by fan-favorite author David Pulver (co-author of GURPS Fourth Edition), GURPS Banestorm: Abydos gives full details on the City of the Dead. This book describes Abydos' history, its religious and temporal organizations, and key features within the city itself. It also includes descriptions of major people and beings of the city, as well as adventure ideas and plot hooks for incorporating Abydos into your campaign.
Although designed for the Banestorm setting, GURPS Banestorm: Abydos is suitable for any GURPS Fantasy campaign that needs a "realistic" city of necromancers and their minions.