It was eventually published not by Addison-Wesley, the publisher of
APUE, but by Prentice Hall and not by the name APUE Second Edition but
Solaris Systems Programming. Presumably he was unable to get AW, or the
estate of WRS, or whoever was the "process group leader", to sign off on
the project and had to shop it elsewhere. So I suspect - and note that I
have no personal knowledge of any people or organizations involved -
that when much of the work on this book was done it was intended to be
APUE II.

Seen in that light, there's nothing wrong with using a similar
organization, writing style, and diagrams; all second editions bear a
striking resemblance to their predecessor.  I agree he should have have
been more explicit about the debt owed to APUE but the charitable
interpretation is that this was originally expected to be obvious and
then the explicit atributions were forgotten as it was scrubbed up for
Prentice Hall

Even beyond all that, APUE is widely considered one of the best-written
technical books ever. Is there some reason subsequent authors shouldn't
stand on the shoulders of giants? In fact what makes Unix so great is
that it's promiscuous in the best sense of the word; it's never been
afraid to borrow ideas. Windows is an original creation; Unix is an
evolutionary mishmash. Which would you rather use? And why shouldn't the
authors if Unix books use the same techniques which worked so well for
the authors of Unix code?

We're not dealing with fiction or poetry here. Technical books should be
judged on true vs false, acessible vs obscure, complete vs spotty. Not
on originality.

--
Henry Townsend