A couple official Green Day songbooks are now available for download from the US iTunes Store, including American Idiot Musical: Guitar Tab, American Idiot Musical: Vocal and Piano, and Green Day: Guitar Tab Anthology which features some of the band's greatest hits from Dookie through 21st Century Breakdown.

There are only those three in the store at the moment but I'd think that they'd be adding more soon. If you're interested in other albums, they have been available in print for a while now and can be found at places such as Guitar Center, Musician's Friend, and Amazon.

Another Green Day related book in the iTunes store is Marc Spitz's "Nobody Likes You" which is a GREAT biography about the band. If you haven't read this yet, I would recommend doing so.