create content > book doesn't go away after disabling module

This is because the book content type is added in book.install, rather than, say, through book_node_info or something. Poll module, for instance, uses poll_node_info to add its content type, while book module uses _book_install_type_create().

I'm not really sure why, but I presume there is a reason book module is doing it this way.

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I'm having the same problem here in v6.3, funny that this core module doesn't act like this in Drupal 5.x

One workaround is to delete the content type manually at /admin/content/types

"create content > book doesn't go away" because book.module doesn't remove that content type upon uninstall. There might be content of type "book" on your website which you don't want to be deleted. If you do, you can remove the content type and then the nodes. I believe that's how this works currently. But I won't close this issue in search of a better solution.

Isn't this the best solution?

Book module creates the content type when it is installed, but doesn't delete the content type on disable/uninstall because it would blindly delete content. This should be the desired solution.

Drupal allows you to manually delete the content type after book module is disabled by going to admin/content/types. Taking this extra step assures that website admin's are purposely deleting content as opposed to it being deleted without the admin's approval.