Does any one can Help improve the methods above

i was told to make an Books Library . i end up with that Code lines . i hope some one could help me to make it better then it is right now , this is mine Code lines of 2 method that i need to improve :

Code:

HashMap<Long,Book> books = new HashMap<>(); @Override public BooksReturnCode addBookItem(Book book) { if(book.getPickPeriod()<3) return BooksReturnCode.PICK_PERIOD_LESS_MIN; if(book.getPickPeriod()>30) return BooksReturnCode.PICK_PERIOD_GREATER_MAX; return books.putIfAbsent(book.getIsbn(), book)==null? BooksReturnCode.OK:BooksReturnCode.BOOK_ITEM_EXIST; } @Override public BooksReturnCode addBookExemplars(long isbn, int amount) { if(books.containsKey(isbn)) { amount++; return BooksReturnCode.OK; } return BooksReturnCode.NO_BOOK_ITEM; }