Hi Aliona, "The reader's book was damaged." is correct and "Reader's book was damaged." is incorrect. Hope that helps.

noun+'+s = possessed by what follows. the reader's book = a book belonging to the reader., whoever they are. reader's book= a book belonging to somebody called named "reader" if you saw said or wrote reader's book, be careful to make it clear that you are referring to a person called name reader. Because there are indeed some peopled called or named reader. the first sentence needs an article "the" in English. the second sentence does not, but only so long as you mean a person called or named "reader". "a reader's book" = can be ambiguous does it mean a book belonging to "a reader?" or something different? "Reader's book was/is damaged" is correct if you mean Mr Reader or John Reader etc.

Looks like you already know the correct sentence, but you ask "why". Some languages need an article, some do not. I don't know the historical reason, but I can explain like this: if you say just "reader" (apart from sounding wrong), then I would be thinking "which reader? a reader I know about? all readers?" To me, the idea is not complete. Hence it needs "the" (a specific reader), or "a" (any reader) or plural "readers' books are..." (many readers or all readers in general) Interestingly, some flavours of English DO say "reader's book..." (without an article) e.g. Indian English. This one reason why Indian English sounds "wrong" to British English speakers. (Although this is perhaps rude, Indian English is just a different dialect and therefore not wrong!) This difference might be due to influences from Hindi or other Indian languages.