Preferred mode kills node/add/book

Site configuration:

* english plus 3 more languages
* a series of book pages in 2 books in english
* a few book pages in other languages

Problem

node/add/book times out

Reason

book_tree_recurse() comes back late or never. Culprit is a very long result from the query in book_toc():

SELECT n.nid, n.title, b.parent, b.weight FROM {node} n INNER JOIN {book} b ON n.vid = b.vid WHERE n.status = 1 ORDER BY b.weight, n.title

The query is rewritten by i18n and it seems that book_tree_recurse() can't handle the result of it.

I didn't go deeper into this, because I think there is a more fundamental conceptual problem here: while 'preferred' mode makes sense for views of nodes or similar, it doesn't make sense within a book. A user shouldn't be able to ever attach a book page of the current language to another language.

I'll be right back with a first shot at a patch for this.