I try this very basic hack

Go to the thunderbird profile
cd ~/.thunderbird/1234wxyz.default/

Find out which address book is which
For me, abook.mab is the Personal Address Book, while abook-1.mab is the google one.

Backup the Personal one: cp -a abook.mab ~/abook.mab.sav

Symlink google to personal: ln -sf abook-1.mab abook.mab

The result is that you actually only have two address books in thunderbird, but just one file on disk. Unfortunately, changes are not directly written into the mab-file so that if you modify anything in the personal address book, the changes will be lost.

What I'd prefer to have is just one address book at all in thunderbird. If anyone knows if this is possible, even by hacking around in the prefs.js and such, please let me know.