Might be worth checking what would happen if such an email was rejected from one of those email servers that bothers to send a response back - could explain it all.
I would say to check the header of your spam e-mail on your friends mail box and see where the e-mail came from, if it was from google you might have some issue (maybe you gave some app or service access to your gmail, or have pop/imap enabled), if its not from gmail then dont stress about it, sooner or later the spam filters will pick up that its not from you ^_^ .oO( yeah dont hold your breath on google helping you on this, they really dont have any support! )
I also suspected it may be some app or service that I gave access to my contacts to, but I didn't see any in my app access list.
How does one get hold of the address book without hacking into the account or via a third party app that has access?
Could even be that it was someone you know who has your email address who gave their contact list to this spammer (directly or indirectly). The bad guys could then have simply picked one email address as the from and got to it spamming the rest.