TSI has a good collection of white papers and EDI links
Sterling Commerce also hold EDI Standards.
Data Interchange PLC are market leaders in OFTP and ODETTE communications and have a good resource on this subject.

For EDI testing s/ware check: Foresight Corporation.

EDIFACT Directories can also be found at the United Nations Website.

The Electronic Commerce Association (UK) have some specialist EDI interest groups

Publications

Electronic Commerce World