BeOS (and now Haiku) implements e-mail like this: the list of e-mails is just your file browser; Haiku's "attributes" provides e-mail specific columns. Opening one of those files launches a viewer app. (Usually 'Mail' but the user can change it)
Your address book? Also just a directory, with address-book-appropriate "attributes."
Your music library? Same
I think it's tragic that we've abandoned this kind of convergence to work within a powerful hierarchical data-table browser widget.