Can this be solved
I dont think any of the currently thought of ideas are actually going to solve this - so it may need some novel thinking.
In a slightly less punnish manner:
"Filters mostly defeated spam for us, “nofollow” has made comment spam next to useless; but what will be able to stop such book spam?"
Filters may have defeated spam for the users, but it is still there and comment spam - while useless - is still on the rise. Neither problem have actually been stopped yet so is it reasonable to think that stopping book spam is economically possible?