@Override
	public String storeBook(MutableBook book) throws InvalidBookException {
		String isbn = book.getIsbn();
		if (isbn == null) {
			throw new InvalidBookException("ISBN is not set");
		}
		this.booksByISBN.put(isbn, book);
		String category = book.getCategory();
		if (category == null) {
			category = DEFAULT_CATEGORY;
		}
		if (this.categories.containsKey(category)) {
			int count = this.categories.get(category);
			this.categories.put(category, count + 1);
		} else {
			this.categories.put(category, 1);
		}
		return isbn;
	}