By default WP seems to display all posts of all types in the main RSS feed. You have to add a post_type query variable to show post of a certain type...
So how can I turn this:
http://site.com/feed/?post_type=book
into
http://site.com/feed/books/
?
And if possible rewrite http://site.com/feed/
to http://site.com/feed/blog/
, which should display only normal posts...