Tim Berners-Lee linked data design note.
A brief and readable note on linked data from TBL. From 2006, but this makes a nice primer on linked data.
Four rules for linked data:
1. Use URIs as names for things
2. Use HTTP URIs so that people can look up those names.
3. When someone looks up a URI, provide useful information, using the standards (RDF, SPARQL)
4. Include links to other URIs. so that they can discover more things.