What steps will reproduce the problem?
1. Install both Auc-Advanced and ProfessionBook AddOns
2. Log into character
3. Error at line 123

What is the expected output? What do you see instead?

An error is logged rather than no errors.

What version of the product are you using?

ProfessionBook 3.0.2 (r72) and wow 3.0.3

Do you have an error log of what happened?

[2008/12/21 04:48:20-285-x2]: ProfessionsBook-3.0.2\PB_Main.lua:123 attempt to index global 'AucAdvanced' (a nil value)

Please provide any additional information below.

Simply adding Auc-Advanced to the list of OptionalDeps in the TOC file ensures that Auc-Advanced is loaded before ProfessionBook.

It would also appear that the use of IsAddOnLoaded to check for an addon is not sufficient to guarantee that an addon has been loaded rather than simply marked for loading.  (This may be a bug in WoW, but is simple to work around.)