David-
I found that the Certification Manual for Welding Inspectors (CM:2000) was extremely helpful in my studying for the exam. It covers a broad range of topics and material that is directly related to D1.1 and has questions at the end of each chapter that are similar to those given for the closed book section of the CWI exam.
The WIT book is similar to the CM:2000 and is also a good book to study. There is a workbook that goes along with it with similar questions as those on the test. I chose the CM:2000 over the WIT because the WIT book is handed out at the seminar, and I didn't want two copies of the same book.
Also, AWS sells a book (CCRM:2006) titled Code Clinic, it's a study guide for D1.1 2006. It's directly related to the code book. I found it helpful in the sense that it has questions related directly towards D1.1 similar to the questions on the exam. I wish I had this prior to attending the seminar, because it would have given me a better understanding of what the test questions were like.
Jason
Jason