The following information is directed towards prospective writers, editors, and publishers of Perl books in the hope that we might see better books out there. Within each category, the points are listed in general order of importance.
Nothing is more crucial in a technical book than technical correctness.
But see the next major point below. If the author is not one of the Perl development team, then have someone who is do a serious tech edit. For a list of likely candidates, check the acknowledgements in the Camel, or see the Perl5-porters mailing list. If they don't know who, what, and where both of these references are, they shouldn't be writing a Perl book. Have someone else on your staff make sure each and every example runs as written, and on at least two different platforms if not more.