Invisible books! What a space saver! They'd be light, no building foundation problems. Clean, wouldn't collect dust and book mold. But what would we do with all that bare wall space? And we'd have to have furniture to fill the rooms instead of books. Ah well. Living around book mold and dust is a good thing. The current thinking is exposure to dirt, cats, dogs, aka as allergens, makes for healthier children and adults (to an extent). I'm living proof of this having been raised surrounded by books (and their accompanying microscopic critters) and cats and dogs. The only thing that occasionally bothers me (allergen-wise) is all the pollen in the air (it's Spring!). But, I take quercetin for that. Works great. Take it whenever I feel the sniffles approaching.
(photo of grounds at Norton Art Gallery)