javaprepare.com – XML certification books
IBM recommends these books for their XML certification exam (exam 141). Since the exam is considered quite difficult, if you can afford them, it might be a good idea to buy all the three books. The books will also act as good reference books for XML. But keep in mind, that none of these books are specifically written for XML certification, so you need to refer to the objectives besides the books.
Professional XML 2nd Edition by Mark Birbeck et al. If you want to buy just one book for XML certification, you should buy this book.