bindFromRequestでnullを返される

Play Frameworkの2.5.10を使用しています。
モデル、フォームの実装をしてコンパイルエラーは出ていない状態なのですが、フォームに値を入れて送信ボタンを押しsaveアクションを実行すると、バインドしたリクエストが全てnullで返されてしまいます。

create.scala.html

html
1@(bookForm : Form[Book])
2
3@import helper._;
4
5<html>
6    <head>
7        <title>
8            新規登録
9        </title>
10    </head>
11    <body>
12
13        <h1>新しい本を登録</h1>
14        @helper.form(action = routes.BooksController.save()){
15            @helper.inputText(bookForm("id"))
16            @helper.inputText(bookForm("title"))
17            @helper.inputText(bookForm("price"))
18            @helper.inputText(bookForm("author"))
19        <input type="submit" value="送信">
20        }
21
22        <!--<form action="/books/create" method="POST">-->
23
24        <!--</form>-->
25    </body>
26</html>

BooksController.java

java
1package controllers;
2
3import play.mvc.Controller;
4import play.mvc.Result;
5
6import views.html.books.*;
7
8import java.util.HashSet;
9import java.util.Set;
10
11import models.*;
12
13import play.data.*;
14
15import javax.inject.Inject;
16
17public class BooksController extends Controller{
18
19    @Inject
20    FormFactory formFactory;
21
22    public Result index(){
23        Set<Book> books = Book.allBooks();
24        return ok(index.render(books));
25
26    }
27
28    public Result create(){
29        Form<Book> bookForm = formFactory.form(Book.class);
30        return ok(create.render(bookForm));
31    }
32
33    public Result save(){
34        Form<Book> bookForm = formFactory.form(Book.class).bindFromRequest();
35        Book book = bookForm.get();
36        Book.add(book);
37        System.out.println(book.id + book.title + book.price + book.author);
38        return redirect(routes.BooksController.index());
39    }
40
41    public Result edit(Integer id){
42        return TODO;
43    }
44
45    public Result update(){
46        return TODO;
47    }
48
49    public Result destroy(Integer id){
50        return TODO;
51    }
52
53    public Result show(Integer id){
54        return TODO;
55    }
56}

Book.javaのモデルです

Java
1package models;
2
3import java.util.HashSet;
4import java.util.Set;
5
6public class Book{
7
8    public Integer id;
9    public String title;
10    public Integer price;
11    public String author;
12
13    public Book(){
14
15    }
16
17    public Book(Integer id, String title, Integer price, String author){
18        this.id = id;
19        this.title = title;
20        this.price = price;
21        this.author = author;
22    }
23
24    private static Set<Book> books;
25
26    static {
27        books = new HashSet();
28        books.add(new Book(1, "C++", 10, "ABC"));
29        books.add(new Book(2,"Java", 20, "DEF"));
30    }
31
32    public static void InvalidData(){
33
34    }
35
36
37    public static Set<Book> allBooks(){
38        return books;
39    }
40
41    public static Book findById(Integer id) {
42        for (Book book : books) {
43            if (id.equals(book.id)) {
44                return book;
45            }
46        }
47
48        return null;
49    }
50
51    public static void add (Book book){
52
53        books.add(book);
54    }
55
56    public static boolean remove(Book book){
57        return books.remove(book);
58    }
59}