The EBook async API is of this form:

guint
e_book_async_get_contact (EBook                 *book,
			  const char            *id,
			  EBookContactCallback   cb,
			  gpointer               closure)

For a long time I thought that the guint return value was an operation ID.  Then the API docs were updated and I see this on every async method:

 * Return value: %TRUE if the operation was started, %FALSE otherwise.

Surely the return value of the async method calls should be a gboolean...