Where can I find programming specification documents?

Check the Important Web Links section.

The best document (arguably) for GBC development information is the abc409 document. Any information that you can not locate on the GBC is usually always exactly the same as information for the older GB series. As a result, use the document described next as a GBC reference as well.

The best document for GB development is the updated Pan/kOOPa document (otherwise known as the GB Manual.)