ISO OSI standards page
This site lists OSI standards by reference number. Look here to find out which standard you need, but you can't download it - the standards must be purchased from the ISO.
OSI: The Network Layer
This page, from Cisco Systems, provides an illustrated overview of OSI with information on OSI network-layer services and protocols, routing protocols, and a description of Cisco's OSI implementation.
Understanding OSI
Provides chapters of a book that explains why OSI is the shape it is and offers critical examination of the OSI approach to specifying rules for computer communication (computer protocols).
