As for the reason only string can be stored in localStorage, an object need to be converted to string format (JSON text).

Save object

var books = [{id:3, name:"From Zero To One"}, {id:4, name:"Head First Java"}];
localStorage.setItem("books", JSON.stringify(books));

Load object

var booksJSON = localStorage.getItem("books");
if( booksJSON == null ){
    // No data stored in the localStorage
    return;
}
var books = JSON.parse(booksJSON);
for(var i=0; i < books.length; ++i) {
    var book = books[i];
    var id = book.id;
    var name = book.name;
}