Are published books to be considered an official reference for spelling? [duplicate]

There is really no "official" source for the English language, but there are sources that are generally well-respected and well-accepted. For spelling, that would be the common dictionaries - Collins, Merriam-Webster, Oxford, etc.

While one would hope that all book publishers would hold their books to the same standards, mistakes slip through and some authors might deliberately mis-use/mis-spell words for effect. So I wouldn't trust any old book to be an authoritative source for spelling.

In your particular case, sorry is considered an adjective in every dictionary I checked, so a plural form - be it sorrys or sorries - doesn't make any sense. I understand how people use it, but it's a made-up word so spell it however you like.

One must distinguish between English grammar and English spelling, which is modern technology (naturally, therefore, full of bugs, since "modern" in this case means 17th-Century).

It's easy to see how one might need a plural form for any English word, since vocabulary isn't inflected and can function as many different parts of speech. So, the buyer for the toy department writes an email to the distributor and says

OK, we'll take 2 dozen Sorries and 3 dozen Monopolies.

or the person taking reservations for a Big Event writes

We've got 83 yesses, 55 maybes, and 44 sorries.

It's quite true that there is no international standard for English spelling (what would be its ISO number?), but there are plenty of dictionaries, though they don't always give the information one's looking for.

In any event, "correct" spelling is really only of use to current search engines. By the time they get better, which will be soon, anybody will be able to spell English any way they want -- which is in fact the current situation -- and still be accessible to search.

There's no need to "back up" a spelling decision. Spelling is just speech representation. Your speech is either understood or it's not. That's all; you pays your money and you takes your choice.