To define a method on Library Books to change the state of a selection of books, you need to add the following code to the model definition:

1. Add a helper method to check whether a state transition is allowed:

    @api.model 
    def is_allowed_transition(self, old_state, new_state): 
        allowed = [('draft', 'available'), 
                   ('available', 'borrowed'), 
                   ('borrowed', 'available'), 
                   ('available', 'lost'), 
                   ('borrowed', 'lost'), 
                   ('lost', 'available')] 
        return (old_state, new_state) in allowed 

2. Add a method to change the state of some books to a new state that is passed as an argument:

    @api.multi 
    def change_state(self, new_state): 
        for book in self: 
            if book.is_allowed_transition(book.state, new_state): 
                book.state = new_state 
            else: 
                continue 

3. Add a method to change the book state by calling the change_state method:

    @api.model
    def make_available(self):
        self.change_state('available')

    @api.model
    def make_borrowed(self):
        self.change_state('borrowed')

    @api.model
    def make_lost(self):
        self.change_state('lost')

4. Add buttons in the <form> view. This will help us trigger these methods from the user interface:

<form>
...
    <button name="make_available" string="Make Available" type="object"/>
    <button name="make_borrowed" string="Make Borrowed" type="object"/>
    <button name="make_lost" string="Make Lost" type="object"/>
...
</form>

Update or install the module to make these changes available.