The Exterior Gateway Protocol (EGP) specifies an EGP that is used to exchange reachability information between routers of the same or differing autonomous systems. EGP is not considered a routing protocol since there is no standard interpretation (i.e. metric) for the distance fields in the EGP update message, so distances are comparable only among routers of the same AS. It is however designed to provide high-quality reachability information, both about neighbor routers and about routes to non-neighbor routers.
EGP is defined by [ROUTE:6]. An implementor almost certainly wants to read [ROUTE:7] and [ROUTE:8] as well, for they contain useful explanations and background material.
The present EGP specification has serious limitations, most importantly a restriction that limits routers to advertising only those networks that are reachable from within the router's autonomous system. This restriction against propagating third party EGP information is to prevent long-lived routing loops. This effectively limits EGP to a two-level hierarchy.
RFC-975 is not a part of the EGP specification, and should be ignored.