Think of 'every' as a word that addresses each and every individual within a group. Although if you change it to 'all,' the meaning can stay the same but the singular becomes plural. Here's a few examples:
Every book is paperback.
Every one of the books is paperback.
All of the books are paperback.
Each book is paperback.
Each of the books is paperback.
Each and every book is paperback.
These examples can have different meanings though. "Every (something) is" can mean you're referring to all of a certain group (for example, all the books in your library), or every single (something) in the world. Hopefully this didn't raise more questions. ;)