question

[osmium]

All I have is K&R1. I did try to get the second version, but due to false
advertising (which in turn was due to both K&R's *apparently* having the
same ISBN), I recieved the first. $20 I'll never get back.

Sounds to me like your book guy tried to convince you that Prentice Hall was
a victim of the massive ISBN famine of 1988.

Here are a couple clips from the ISBN Gods.

What is an ISBN?
The International Standard Book Number (ISBN) is a 10-digit number that
uniquely identifies books and book-like products published internationally.

What is the purpose of an ISBN?
The purpose of the ISBN is to establish and identify one title or edition
of a title from one specific publisher and is unique to that edition,
allowing for more efficient marketing of products by booksellers, libraries,
universities, wholesalers and distributors.
http://www.isbn.org/standards/home/isbn/us/isbnqa.asp#Q1

Note that the word "uniquely" plays a key part in each paragraph.