The count of items and the count of people
In the lesson 4/5 of chapter "Numbers", I found that: She has a thousand books - Ella tiene un mil libros. I have a million friends - Tengo un millon de amigos.
There is a "de" ahead of amigos but not one ahead of libros. Is that the difference between count of items and the count of people (or saying living creature)?
My grammar book says millon gets a 'de' after it when used before a noun. I think mil, when made plural as miles, also gets a 'de' after it. Similiar to English. I have a thousand books, I have thousands of books.