Also these include law books and encyclopedias which are out of date. I don't know how they used to do with encyclopedias, but law books generally get slowly more and more out of date as laws change and new cases are decided by appellate courts. You get a packet to stick in the back which adds to or modifies the contents, and eventually the packet is big enough that they stop issuing new ones and instead you have to spring for a new set of books.
A full set of cases for a state and that state's statues could easily be two yards of books right there.
Then there's all the books that are shit and everyone knows it but the publisher thinks they can sell a few. They end up running off too many and offload the remainder. Derivative cookbooks, biographies for flash-in-the-pan celebrities, books ghostwritten for people you won't care about next year and probably don't know about now, religious and political screeds with all the literary weight of a joke on a popsicle stick. These are wastes of paper when they're new, and worth less than their weight in firewood after a couple years.
Then there's the books that were decent when they came out but are no longer relevant. Atlases with USSR on them, repair manuals for vehicles nobody drives because they're too old to be good anymore but not cool enough to restore, Dummies guides for things everyone knows how to do now, tech manuals for operating systems nobody uses at home, chemistry textbooks from the 50s.
Then there's all the niche books, of good quality but very limited interest. Maritime history, commercial aircraft identification, yet another Crimean War history, diseases and infirmities of the
pangolin (which are adorable and I totally want one), butterfly biology, field guides to minerals for places nobody goes, local history series for places where people suffer alienation.
Sad as it is, sometimes there are just books that nobody wants to read anymore.