I got you.

The article addresses this

The shelves themselves are a history of optimism. The first ones were beautiful — solid oak, custom-built, spaced to accommodate the quartos and folios that seemed, at the time, to represent the future shape of the collection. They were expensive, and they filled up in eighteen months. The second wave was IKEA Billy bookcases, deployed with the pragmatism of a military logistics officer: cheap, modular, immediately available, and — this is their great virtue — exactly 28 centimetres deep, which accommodates 95% of octavos and all paperbacks. The third wave was industrial steel shelving in the basement, the kind used in warehouses. It is ugly. It holds a lot of books. At a certain point in a collector's life, capacity trumps aesthetics.

You can double-shelve (books in front of books), which hides half your collection behind the other half and makes finding anything an archaeological expedition. You can stack horizontally on top of vertical rows, which looks terrible and eventually causes the shelf to bow. You can put books in boxes, which solves the space problem by creating a different problem: you now own boxes of books instead of a library.

I have done all of these things. I am not recommending any of them.