edit book nodes - which function take care of?

Hi all,

I'm working with drupal and some different node types in my site.
I have a unique problem with book nodes-

usually, when editing an existing node, the "theme_node_form" function is called (and then call the rendering function ).
BUT-
when I give a path: node/$nid/edit and the node type is book, I arrived to the rendering function but not through the "theme_node_form" function... when I create new book node - it works o.k (using the "theme_node_form" function)

Do you know what is the different behavior of book???

thanks in advance!
Miriam.