I'm using 'Now Reading' wordpress plugin inside a wordpress project. In plugin's sidebar template I'm by using this query: while( have_books('status=read&orderby=finished&num=2') ) : the_book() to choose 2 books. What parameter must i pass to really make it random? I attempted with 'order=rand' and 'rand=true' but it didn't work. Any assistance will be appreciated! Thanks ahead of time..

A random function does not really exist. I have tried personally this code, during my styles functions.php to permit random order before - unsure if it'll work in cases like this, but worth an attempt.

Add this for your styles functions.php file:

function query_random_posts($query) {
    return query_posts($query . '&random=true');
}
class RandomPosts {
    function orderby($orderby) {
        if ( get_query_var('random') == 'true' )
            return "RAND()";
        else
            return $orderby;
        }
    function register_query_var($vars) {
        $vars[] = 'random';
        return $vars;
    }
}
add_filter( 'posts_orderby', array('RandomPosts', 'orderby') );
add_filter( 'query_vars', array('RandomPosts', 'register_query_var') );

Then do this inside your sidebar file:

while( have_books('status=read&orderby=finished&num=2&random=true') ) : the_book();

Otherwise, my only other suggestion is always to obtain the 10 latest books, add all of them to a different array, after which shuffle that array. Might be a little bloated though.