New node - new book root

Which ever content-type you create from the 'create content' menu, they always start at as the Book parent. Or at least that's how I remember it.

It's only when you visit a book page, and then click 'add child page' that the books parent is set in the node/add form.

But to answer your question, some simple hook_form_alter() function magic in a custom module should allow you to default the parent selector depending on the content-type name. You may even be able to get away without writing any code, and instead using the form alter module available here on d.o.

I've not used Drupal 6's Book/outline system so am just going off Drupal 5 and 4.7 on the above.

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I've created a module (BookMadeSimple) which can do this.

1. You can automatically create a main book page for any content type (customized in admin section)
2. It adds a dropdown listbox in book links to add any child content. Selecting the content type in the dropdown list box will add it as child page

For more explications, see BookMadeSimple home page