Published by the Jordan Tourism Board, the free Jerash brochure includes a map, some photos and a recommended walking route. It can be found at the visitor centre in Jerash.
Anyone with a particular interest in the history of Jerash should pick up one of three decent pocket-size guides: Jerash: The Roman City, Jerash: A Unique Example of a Roman City or (the most comprehensive and readable) Jerash. All three are available at bookshops in Amman. Jerash by Iain Browning gives a more detailed historical account.
Near the South Gate, the visitor centre has informative descriptions and reconstructions of many buildings in Jerash, as well as a good relief map of the ancient city. The site's ticket office is in a modern souq with souvenir and antique shops, a post office and a semi-traditional coffeehouse. Keep your ticket, as you'll have to show it at the South Gate.
Toilets are available at the site entrance (inside the souvenir souq), at Jerash Rest House and at the visitor centre.
It’s possible to leave limited luggage at Jerash Rest House, for no charge, while you visit the site.