I began professionally reviewing books in the early 80s. Over the years I have seen a steady decline in the quality of both orthography and proof-reading. At least one book, privately printed, was almost unreadable because of it.

Part of the problem is that costs are rising and often proofing can be air tasked or subbed out to freelancers, who don't have the experience, but are cheap.

At the very least, no book should have any error other than grammar or 'literals' (words spelled correctly, but the wrong word - their for there and so on) now we have spell-checkers.

But often, and increasingly, that's not true.

Just another reason to love old books.