It’s a sad commentary on historical sources, but old encyclopedias and similar sets are of no monetary value whatsoever. At our library’s annual book sale, we often receive donations of complete, perfect, pristine sets. We try to discourage them, because they are sent right to the dumpster. You can’t pay anyone to take them other than trash haulers.
100 years from now, I’m pretty sure things will change, and we’ll all be sorry.
Maybe the best thing to do is to make some kind of paper-friendly time capsule and bury them for future archaeologists. Air-tight with lots of desiccant, perhaps.
Rather than doing that, I would make things with them. They are great for making hollowed out books. You can hollow them out and decorate the covers.
You and Chrome Toaster are totally out of the running.
I am sure this is a valid utilization of an otherwise useless set of books - just not my copy. 
Again, if I were actually capable of mutilating them, I could send them to recycling, which in fact is what I am doing with my yearbooks today.
ETA: Is there a market for hollowed out books (decorated by a non-crafty person such as myself)? Because otherwise, they are still in my house.
Woo!
My church has an informal free library, run on the basis of “take a book if you like, bring it back if you like, or bring in a different book”, and while we get encyclopedias donated every so often, we can never hold on to them. They always get taken out and not returned, so there must be someone out there who wants them. Oddly, though, it’s always just one volume at a time. My best guess is that someone’s kid has a school project due on some subject or another, and the teacher put a limit on how many of the required sources can be webpages.
And I never read the Britannica cover-to-cover, just randomly… But our set also came with a set of Great Books, and I did read several of those straight through, including Euclid’s Elements. I was especially fascinated whenever I came to a proof that I knew was wrong, to see just how Euclid had screwed up.
Half Price Books takes old encyclopedias. They donate them to libraries in poorer countries.
Huh. There is one within my travel range. I have emailed them to verify.
Is it really helpful to send someone desperate for knowledge a set of books declaring that, one day, man will go to space? I mean, we might find that charming, but it kind of feels like sabotage.
Update, in which my Encyclopedia Britannica met an ignominious end.
Half Price Books sounded like the best available option. There isn’t one nearby, so I waited until I had to go to the Bay Area anyway. It was still out of my way, and driving 680 south of Benicia was was one the more harsh, joyless routes I have had the displeasure of traveling.
Since encyclopedia are so hard to get rid of, I had taken the precaution of emailing HPB ahead of time just to make sure, and they said, yes, we take them, we’ll make you and offer.
You can imagine my disappointment when I arrived only to be told that NO, they do not accept encyclopedias. They recommended I drop them off at the nearby Savers (a large chain thrift store) which I did. Heck, I could have done that in my own town.
Some very cool ideas here on how to reuse those old encyclopedias:
I had a full set of late '70s Encyclopedia Britannica (micropedia, macropedia & several year-books) that I brought about 10 years ago to an Internet consigner to sell on eBay. As I recall, my check for 75% of the proceeds was about $5!:eek: I presume someone bought them for “window dressing” an office or the like rather than actually reading them. 
Yeah, I wasn’t expecting to make any money off of them; I had only wanted the illusion that they might be going to a good home.
Update: I forwarded the email in which HPB books had stated they would make an offer (which they didn’t) to customer service. I did receive an apology and an offer of a gift card.