How to use multiple iOS custom view controllers without a navigation controller

I am building an app that uses multiple types of screens--all which warrant their own custom view controllers. I am successfully switching between view controllers and their related views by reassigning the main window's rootViewController with a method in my app delegate like the following:

- (void)changeRootViewController:(NSString *)controllerName
{
    if (controllerName == @"book") {
        rootViewController = (UIViewController *)[[BookViewController alloc] init];
        [self.window setRootViewController:rootViewController];
    } else if (controllerName == @"something_else") {
        // Use a different VC as roowViewController
    }
}

The way that I am doing this seems like it just can't be best practice, however. I don't want to use a UINavigationController or a UITabBarController as the rootViewController, either. Is this the wrong way to be doing this, and if so, how should I be approaching this differently?

I thought this would have been covered somewhere, but (I feel as if) I've Googled the heck out of it, looked for related questions, etc. Sorry if I've missed something!