Get Author for Single Post in Custom Post Type

There are several functions for this available, like: the_author, for displaying; get_the_author, for returning. There is no restriction regarding custom post types, but those template tags only work inside the loop.
If you want to get more author information then the name you can use: the_author_meta, displaying; get_the_author_meta, returning. The latter two functions can also be used for getting author information outside the loop, for this you have to specify the second parameter $userID, which inside the loop isn’t necessary. A basic example for this would be going like this:

global $post;
$a_id=$post->post_author;
the_author_meta( 'user_nicename', $a_id );

Take a look at this question and the answers for more outside the loop use cases.