I run a Drupal 7 website since a year and have been always upgrading it - up to the version 7.10.
I have 2 Books and users' Blog at my web site. One of the Books is holding "Site Help" and "Imprint" stuff.
I go to /node/add but there is a message that no "material types" have been defined.
But I don't remember deleting them. And when I go to /admin/structure/types I see both "Machine name: blog" and "book" defined there: