Is this change (was-> were) only applicable for the first person and not for the third? No, it applies to all persons. Is the book wrong? It's not wrong. We do say '..if it wasn't so expensive'. But I think the book is misleading you if it suggests that this is the correct form, or the only correct form which we use. Strictly speaking, it should be 'if it weren't', but it is very common indeed for native speakers to use the 'was' form in is place. Are both sentences correct? Yes, in a way. Only the most pedantic of grammarians would say that 'if it wasn't' is incorrect. It's simply that 'weren't' is more correct.

The correct sentence would be the one with were, but in everyday conversation was is often used, that's why it can be found in different books.

Ya ivAna is right. I want to add one more thing. If there was possibility to happen things, so we can use" was ". Let's take another example :-" if I were bird I would fly". In this example it's totally impossible thing to happen so we use "were". So that's the different however English speakers use "was" instead of were. I'm English learner. If I am not correct, Pls correct me.

I think you hear both interchangeably in conversation, so either way to say it you'll sound fine. Technically (and the book is being technical) it's right. "It were" would not make sense if you change context: "It were hot," "It were cold"— we know those don't sound right. Same with the jacket: It (third person singular) WAS expensive, so I did not buy it. Books right, but conversationally very few people would comment on a small thing like that. Hope that helps.