[Plugin: Quick Post Widget] how to allow inserting relation between posts?

it would be nice know how to allow user to add relation between
custom post type in front-end editor. any ideas?
i would like to have some kind of interface like posts to posts plug-in
we have in administration area.

for example after adding a custom post type book,
i would like to add a list of authors.

thanks to anyone can help

http://wordpress.org/extend/plugins/quick-post-widget/