I got some good assemble-'em-yourself bookcases at Costco a couple of months ago. They have 48" shelves with a center wall for extra support, they ARE in fact 7 ft high, and they have 5 movable shelves each. Priced just under $300 a unit, as I recall -- certainly not more, and possibly less. So that's 72 feet of shelf space, counting the bottom fixed shelf and NOT counting the top. They even look nice, in a light oak sort of way. The visible parts are wood; the unseen bits are very heavy MDF.

My books are now coming out of storage after ten years!