This is a black coat
They are black coats
How much is this pear?
How much are these peaches?
It's an English book
Because the plural of it is they, the plural of is is are, the plural of book is books, and a / an can't be used before plural nouns, so we should remove an, English is attributive to modify noun book, there is no plural, so the plural of this sentence is they are English books