It is good this person can not anymore deprive future generations of readers books and the knowledge they contain.
1. Rape and pillage of settlements.
2. Using child soldiers.
3. Stealing books from libraries.
4. Deploying poisonous weapons on civilians.
So let me get this straight: you believe that stealing books from libraries is the equivalent of, say, massacring civilians during wars?
$5 million can do a tremendous amount of good. It can feed a lot of people, educate a small village from birth to adulthood, save hundreds of lives from Malaria and other mosquito-born diseases, and pay for many other life-saving medical interventions.
Should the library sell the books and spend the money on the above items?
I know my analogy doesn't quite jive with the civilian-massacre-vs-book-thieving choice. But hopefully it shows just how immensely important libraries are. Had the Library at Alexandria not been destroyed, there's no telling how much earlier our civilization would have advanced to the relatively peaceful time we live in now.
I guess the difference between then and now is that we have the ability to digitize all these rare works. That should be one of the highest priorities for our civilization these days. Get them digitized and replicated. There's still the storage medium problems, but with enough replication, we can hopefully ensure the durability of all that information nonetheless.
I wouldn't really mind it if all the knowledge of the library of Alexandria were preserved due to backups located 20 miles away, whilst the old originals burned.
Let's assume the entire contents of a rare book—or let's say a famous renaissance painting—is digitized at high quality. Is it a moral failing for a public library or museum to hold on to the work instead of converting it to cash and spending that cash on life-saving initiatives? Is valuing these original works an irrational thing that we humans do? I can't logically argue for keeping the works, yet I think something would be lost if all art was sold to the highest bidder and the proceeds used for charity.