Pulling custom press post field

Hi Developer,

Just wanna know how to pull a value of a field assign to a custom post type:
I.E. I have a book, and it has a field called "Thumbnail Image"
How do I get the value of the Thumbnail Image?

BTW I am using this code:
<?php echo get_post_meta( $post->ID, 'Thumbnail Image', true ); ?>
but it doesn't working:

Any thoughts?

Thanks