function within a function?

[eluser]livewirerules[/eluser]
Im having a model class called books and a function called horror that will retriev and display all the horror books that are available in the database

the controller can be accessed by http://localhost/index.php/books/horror

Code:

function horror(){

//the code

}

what i want to do is to grab all the books by available,unavailable
so i can access it by http://localhost/index.php/books/horror/available or http://localhost/index.php/books/horror/unavailable

i think its something like calling a function within a function but i dont have an idea how to do it. coz i im learning CI , any help and ideas how to figure this out will be well appreciated.

thanks