Custom post type and URL

Question

In short – you cannot, or at least should not.

www.domain.com/post-title is interpreted as post (if your structure is %postname%) and/or a page – and WordPress is set to deal with them as such, including any conflicts.

You can do the following:

add_action('init','wpse0845_add_rule');
function wpse0845_add_rule(){
    add_rewrite_rule("([^/]+)/?",'index.php?book=$matches[1]','top');
}

(and then flush rewrite rules, once).

However, you’ll notice that now your pages (and depending on your structure, posts too) give a 404.