FIXED! (Bjoern 11:47, 19 April 2009 (UTC)), possibly since http://groups.google.com/group/mwlib/browse_thread/thread/8dcb009eedfaf8a5
The problem: Suppose pages A and B appear in the book definition:
; Book section : A : B
Suppose B is also transcluded into A: I would expect the page to appear twice: Once as part of A, and once as required by the book definition. However the page only appears once: It is only transcluded into A, and doesn't appear as required by the book definition.
book - in this book, a page is both transcluded in a chapter (which then appears on the book definiton page), and also appears directly on the book definition page. Only where the page is transcluded (in a chapter) it appears (as part of that chapter), but it doesn't appear as stated on the book definition page.
book2 - the same book, but this time the page is not transcluded: The page appears multiple times as required by the book definition page.
book3 - here two different chapters transclude the same page: The page appears as part of both those chapters, but does not appear as required by the book definition page itself.
MediaWiki:Books/HCPT includes page, that has already been transcluded: Page does not appear in output.
It seems that sometimes pages included in the book definiton do not appear in the book.
Cf. http://code.pediapress.com/wiki/ticket/454 and http://de.wikipedia.org/wiki/Hilfe:Buchfunktion/Feedback_zur_Buchfunktion#Fehlende_Seiten