[Adamantcheese]

Use one of the solutions listed below, but you HAVE to do sorting on the fly. You need to have places to put books and sort them by some general genres and you HAVE to throw out books that aren't worth the time due to damage or any other reason a book would be deemed a recyclable. With that many books, a proper library style cataloging system may be your best bet.

That being said, if you do want to do image comparison for covers, books without covers usually have a copyright page with most of the info on it. Use that to determine what a book is when the other method fails. Throwing together some cheap bookshelves with plywood and 2x4's will greatly help with the finding part, but while scanning use some big bins to do a rough sort.

And I can't stress enough you HAVE to throw out books. It's clear that there's a space issue and if he's willing to get them for free but has a hard time getting rid of them, that's hoarder behavior, not just eccentricity.