There is a lot of understandable speculation about the value of the content that has been library-sourced in the Google Book Search settlement proposal.  Besides the public domain, lot of the volume is rendered "Not Commercially Available" by definition of the proposal, and therefore uniquely, or nearly uniquely, available through Google Book Search.  Libraries are delighted this content can now be made available digitally.  And that's a great thing, I agree.

However, in terms of revenue generation, I'd trade a whole backlist for the frontlist: e.g., for the 37 percent (Google's share) of the sales of "Asian Adventures: Hot Nights" at $7.99 for consumer access.  Attention spans are short in the human species, and the transition to digital does benefit long tail reading, but it also arguably biases against older texts.  (There are at least two reasons for that bias: older texts are best known by older people, who are less digital; and older books with pre-modern fonts render less successful OCR through high-volume digitization.)

The settlement basis on library- held content assures steady institutional income from libraries etc., at relatively low maintenance costs (heavily front-end subsidized by the participating libraries, who then tax themselves in perpetuity as a class for the opportunity).

That income source aside, the revenue window might be characterized as two fold, direct and indirect.  Direct, moving forward, with in-copyright, in-print (commercially available) that can be digitally distributed through various means, and the secondary marketing options therein available ("Get the Android screen saver edition!").  Remembering: the past is finite and always behind us, but that future content production and sales are presumably less limited, unless we destroy our societies or our planet.

Indirect benefits come from the registry infrastructure and how that plays out, particularly in re: other media channels, as this could move not just into videos and music, but also contemporary (library-unbound) journal and article literature, photography, illustrations, and other sectors only barely touched, or put aside, in the current settlement negotiations.

And, priceless: knowing through Google IDs who is reading Asian Adventures, and other books they are browsing, and what they are hitting on in web searches, and what they are reading in Google News; subscribing to in RSS feeds; what they are looking up in Google Maps; etc. ... That is all pretty nice fodder for advertising and ancillary opportunities.

Notably, only aggregated GBS use-data will be available to the settlement's rightsholders (authors and publishers); none of it is likely to ever be available to institutional or consumer subscribers.