You don't say which bike, but I think the Official Service Manual is the best by far, no matter what machine.
I agree. For the 1500 and 1800 Wings (I'm not sure about for earlier models, and the OP doesn't say what Goldwing he wants a manual for) the ETM also shows photos and locations of all the connectors and relays.If you only get one, go for the OE manual and ETM. The ETM alone is worth the money if you're doing any electrical diagnostics.
The Helm's manuals are licensed reprints of official Honda manuals no longer in print.ONLY buy a factory manual from ebay if the price is considerably less than the Helm manual. It's amazing how many guys buy the Helm manual and then ask double what they pay for them.
Helm charges $65 for the manual. Ebay ones go for up to twice that. Buyer beware...