Why don't you get them down, sort through and pick your very faves out, then sell the rest to buy a lovely bookcase for your bestest most favorites? Books really don't store well over a long period of time, and in the roof they will be exposed to extremes of temp, I'd hate you to get them down in a couple of years and they're wrecked
I would but we already have a bookshelf we don't use in case the kids get them, maybe i should clean off the top 2 shelves and keep some of my faves though, i have most of the enid blyton set i want to keep for my girls plus quite a few others... hmmmmm now you got me thinking, i miss reading