I used "Process Piping - The Complete Guide to ASME B31.3 Third edition" by Charles Becht
Now, this IS a "guide" and NOT the complete ASME 31.3 with all of its appendixes and additions.
On the other hand, because it IS a "guide" it is better than the complete book for studying and understanding what the intent and purpose of the complete ASME B31.3
If I were to take the CWI exam again, I would definitely "study" this book, but bring both it AND the complete ASME Code book with all of its appendixes into the "open book" part of the exam. Get it in addition to the ones above.