I am wandering is it a local policy on en: (and some other Wikibooks)
or, like on Wikipedia, it is a global policy?
Yes, this is reasonable if we are talking only about manuals and
school books. However, I would like to see, for example, a book about
web servers comparison and I don't think that it is reasonable to stop
people writing such kinds of books. Also, I'd like to see essays and
even fiction books on Wikibooks, too. Of course, with defining what
kind of books would be able not to be NPOV and NOR (for example, I
don't think that a book about history may be POV) and with marking
such books as OR and POV.
If it is locally related to en:, projects in other languages may not
follow such rules. However, it would be better to have a global policy
with definitions what may and what may not be OR and/or POV.
I think that it is better to have strong Wikibookian communities with
a lot of fiction on Wikibooks then much smaller communities without
fiction on Wikibooks.
I know that it sounds hereticly :) However, I would like to see a good
skilled (amateur) astronomer who prefer to write SF on Wikibooks. He
may start to write a book about astronomy through some time.