Hi Again Bruce,
First Quit Mail, then I'd backup these two Mail folders, by right clicking on them in the Finder, then choose Archive/Compress.
(Could be a different folder here if you chose such in Mail Prefs)
Right click on that Mail folder, choose archive, you'll get everything in the folder, and the folder itself in a file called Mail.zip, move it to a safe place, same for the Mail Downloads folder... only the plist is separate.
For Address Book, quit AB...