Hi Rachel,
I'm having the exact same issue. I got through all the exercises in the book up until Chapter 17, page 341. When I navigate to my login.php I get:
Warning: require(Auth/HTTP.php) [function.require]: failed to open stream: No such file or directory in /Users/lindsay/Sites/PRACTICE/PHP/CH17/[removed]/login.php on line 5
I tried searching for the file 'Auth/HTTP.php' in my filesystem (through the MAC terminal) which is required on this line (i.e. require_once('Auth/HTTP.php')

and it appears as if this file does not exist in the Pear directory. Does it not come with the basic installation?
BTW - My Chapter 9 exercises that use Pear all work so I know it's not my php.ini file configuration.
--
Updated:
I think that this is one step in the right direction:
http://pear.php.net/.../Auth/download/
but in that download, there's no HTTP.php file, so I'm not sure which file replaces HTTP.php.
--
ANSWER:
if you read the documentation for the newest version of Auth (http://pear.php.net/manual/en/package.authentication.auth.intro.php) you only have to change
require_once('Auth/HTTP.php');
TO
require_once('Auth.php');
It appears as if the code "Auth_HTTP" might not work, so I'm going off to research that.
SO RACHEL - the next edition has to update everything about Pear's Authentication plugin.