Because of copyright issues, countless books between around the 1930s until about 2000 were never digitized.

After around 2000 books started coming out in digital format, so at least there are digital copies of many of those, even if they are still under copyright.

I dunno, that "at least" worries me, digital actually more easier to be lost if it is under copyright, they just got deleted if they can not produce enough money. Physical may have higher chance to survive.