I would buy every single book they give you ahead of time- it's a lot of material  to read and comprehend in a week.
The Certification Manual for Welding inspectors is a good book as it explains subjects better than the WIT in my opinion.
On ebay you can find practice tests for sale. These are very good as well

Here it is. Everything the AWS suggests.
Since your employer is footing the bill, explain to them that all of these will be valuable reference material for your job.

http://www.aws.org/library/doclib/BodyOfKnowledge.pdf

I keep hearing comments to the effect; "I studied 'this' subject or 'that' subject, and it was a waste of time as there was only ONE question on the exam pertaining to this. Or... the instructor neglected to spend enough time on a particular subject from which multiple questions were drawn from.

Contained in the link above is perhaps a million words of technical information that has been recommended for study. The ONE paragraph you fail to read might have a question drawn from that on the exam.

Another thing to spend some time on (THIS will save you valuable time on test day!) is, do you have a firm functional ability to perform 8th grade mathematics (algebra)? Most people it seems these days can't perform well at a 6th grade level.
Now, that being said, there is nothing on the exam that +,-,X and / cannot be solved with a pencil and paper. I forgot my calculator and took the last exam with a large format 4 function calculator borrowed from the concierge's desk. The ol' trusty TI-35 would have been faster, but this simple device did save time over long hand math. My best advice to most candidates is to sharpen your basic math and calculator skills. This will save time to ponder all those questions that "the seminar instructor" failed to properly prepare you for.

You're welcome to do your own price search and comparisons. Amazon could be your friend if needed. I find many reference books on there used for 1/2 price or less.