3.2km of linear storage space makes sense for books. You aren't just piling them up in stacks, where volume might be a useful measure, and you aren't putting th

3.2km of linear storage space makes sense for books. You aren't just piling them up in stacks, where volume might be a useful measure, and you aren't putting them arbitrarily deep on the same row because that prevents access. You'll usually store things like this one book deep. If you have a 4-row shelf where you could have an 8-row shelf with the same width, each row 1m wide, you have 4m vs 8m of linear storage space.