Need to display author’s email id in the “Edit post” field in wp dashboard. How do I do this?

Probably you use get_post on edit page with query object. You can simply use $post->post_author on edit page. For example;

$editableid = get_query_var('id');
$postedit = get_post($editableid);
$author_id = $postedit->post_author; //Always return author ID value.

<input type="hidden" name="post_author" value="<?php echo $author_id;?>" />

I hope this code piece helps you.

EDIT : For email you use the_author_meta with your $author_id value.