What causes boxy artifacts when photographing or scanning old printed book pages?

The pictures you want to copy are “halftone” images. The original was fractured into countless dots of ink that either vary in size or vary in spacing or both.

The halftone method is the mainstay of how pictures are printed on paper with ink. Halftones are made using an assortment of methods depending on the era. You can identify a halftone by examining the image with a magnifying glass. Newspaper pictures are halftones that are easy to see because the paper is inexpensive it absorbs ink allowing the dots to spread, don't need to view with a magnifier. Anyway, halftone images are not solid. they are comprised of dots of ink.

Scanning a halftone presents special problems. Flatbed and document scanners contain scanner cells and halftone screens that fracture the original into a fine grid pattern. Because the original was also a halftone, we get unexpected patterns. These can vary from fine herringbone or crosshatch or moiré patterns. What you are seeing are moiré patterns.

The moiré pattern or effect occurs when viewing set of lines or dots that are superimposed on one another. To see this effect in real life, look through an ordinary window screens looking at background objects with a pattern or better, look through two layers of ordinary window screen