How to bind or inject variables from service providers to model class laravel

I have this class called Book and I want to have one global variable sort of thing in that class which consist of an exchange rate (For whole data/model) I am trying to use service containers to inject the variable in it but I am sort of loss here

I tried this code within AppServiceProvider.php in the register() method, but it doesn't work.

$variable = 'something';
$this->app->bind('App\Models\FrontEnd\Book', function ($app) {
    return new $variable;
});

via Chebli Mohamed