Design data structures for an online book reader system

Design the data structures for an online book reader system.

My initial thoughts:
I design this book reader system based on iBooks from Apple: you can search for books, add bookmarks and so on.

	public class MyBook {
		private final String author;
		private final String title;
		private final int number_pages;
		private final List<String> content;
		private final List<Integer> chapterIndex;

		private List<Integer> bookmarks;

		public MyBook(String author, String title, int number_pages,
				ArrayList<String> content, List<Integer> chapterIndex) {
			super();
			this.author = author;
			this.title = title;
			this.number_pages = number_pages;
			this.content = content;
			this.chapterIndex = chapterIndex;
		}

		public void addBookMark(int pageNumber) {
			bookmarks.add(pageNumber);
		}

		public List<Integer> getBookmarks() {
			return bookmarks;
		}

		public String getPage(int pageNumber) {
			return content.get(pageNumber);
		}
	}

	public class MyOnlineReader {
		private Map<String, Map<String, MyBook>> bookMap;
		private MyBook currentBook;
		private int currentPage;

		public void openBook(String author, String title) {
			currentBook = bookMap.get(author).get(title);
			List<Integer> bookmarks = currentBook.getBookmarks();
			if (bookmarks.isEmpty())
				bookmarks.add(0);
			currentPage = bookmarks.get(bookmarks.size() - 1);
			System.out.println(currentBook.getPage(currentPage));
		}

		private boolean finished() {
			return currentPage == currentBook.number_pages - 1;
		}

		public void addBookMark() {
			if (!currentBook.getBookmarks().contains(currentPage))
				currentBook.addBookMark(currentPage);
		}

		public void displayAllBookMark() {
			System.out.println(currentBook.getBookmarks());
		}

		public void nextPage() {
			List<Integer> bookmarks = currentBook.getBookmarks();
			bookmarks.remove(currentPage);
			currentPage = finished() ? 
					currentPage : currentPage + 1;
			bookmarks.add(currentPage);
			System.out.println(currentBook.getPage(currentPage));
		}

		public void previousPage() {
			List<Integer> bookmarks = currentBook.getBookmarks();
			bookmarks.remove(currentPage);
			currentPage = currentPage == 0 ? 
					currentPage : currentPage - 1;
			bookmarks.add(currentPage);
			System.out.println(currentBook.getPage(currentPage));
		}

		public void goToPage(int pageNumber) {
			if (pageNumber < 0 || pageNumber >= currentBook.number_pages)
				return;
			List<Integer> bookmarks = currentBook.getBookmarks();
			bookmarks.remove(currentPage);
			currentPage = pageNumber;
			bookmarks.add(currentPage);
			System.out.println(currentBook.getPage(currentPage));
		}
	}

Solution:
	public class Book {
		private long ID;
		private String details;
		private static Set<Book> books;

		public long getID() {
			return ID;
		}

		public Book(long id, String details) {
		}

		public static void addBook(long iD, String details) {
			books.add(new Book(iD, details));
		}

		public void update() {
		}

		public static void delete(Book b) {
			books.remove(b);
		}

		public static Book find(long id) {
			for (Book b : books)
				if (b.getID() == id)
					return b;
			return null;
		}
	}

	public class User {
		private long ID;
		private String details;
		private int accountType;
		private static Set<User> users;

		public long getID() {
			return ID;
		}

		public Book searchLibrary(long id) {
			return Book.find(id);
		}

		public void renewMembership() {
		}

		public static User find(long ID) {
			for (User u : users) {
				if (u.getID() == ID)
					return u;
			}
			return null;
		}

		public static void addUser(long ID, String details, 
				int accountType) {
			users.add(new User(ID, details, accountType));
		}

		public User(long iD, String details, int accountType) {
		}
	}

	public class OnlineReaderSystem {
		private Book b;
		private User u;

		public OnlineReaderSystem(Book b, User u) {
		}

		public void listenRequest() {
		}

		public Book searchBook(long ID) {
			return Book.find(ID);
		}

		public User searchUser(long ID) {
			return User.find(ID);
		}

		public void display() {
		}
	}

One thing I learnt from the solution is to put a private static set of all users in a user class. Hence all of the single user shares a copy of references to all the other users. Brilliant design.