I just came across David Lizarraga's previous post where he recommended the following:
1) Java Web Services: Up and Running - Martin Kalin
This should be the first book you read to get a hand of what are soap and rest web services, what can you do with them and how to implement them with JAX-WS and JAX-RS. If you are completely new to this topic I recommend writing some of the code examples the book gives, and if you already have some knowledge you can just go through the book skipping the parts that you consider you already know well enough.
2) OCEJWSD 6 study notes - Mikalai Zaikin http://java.boot.by/ocewsd6-guide/ This one is a must. It will be your first source of knowledge for the exam and it covers most of the topics of the exam.
3) Version 5 of the exam study notes - Ivan Krizsan http://www.ivankrizsan.se/my-books/ Version 5 of the exam study notes - Mikalai Zaikin http://java.boot.by/scdjws5-guide/ What was not covered in Mikalai Zaikin version 6 of the exam notes, you will find it in one of these two resources. They are targeted to the Java EE 5 web services exam, but most of these topics are included in the version 6.
4) OCEJWSD 6 Mock Test - Mikalai Zaikin
This mock exam simulates the multiple choice style questions you will find in the exam and it includes explanations for every one of the possible answers. Thanks to this explanations, you can use it not only near the end of the preparation but also as a study method when you get tired of just reading. This one is not for free but the price can be considered symbolic for the value it actually has. Send and email or an MP to Mikalai and he will tell you how to buy it.
5) Enthuware OCEJWSD 6 Mock Exam - Question bank created by Fritz Walraven http://enthuware.com/index.php/mock-exams/oracle-certified-expert/oce-web-services-mock-questions The last 3-4 weeks you should spend them doing this test. It includes lots of questions that resemble the real exam and some of them are even harder (if you pass this exams without cheating you will probably pass the real one), and it will help you with getting used to the exam format. The question bank was created by Fritz Walraven, which if you are here you probably already know, but if you don't, he is a rancher here who always helps solving the web services questions in this forum. The software has a nice feature that consists in that when you are answering a question, you will have a "Discuss" button, which will take you a topic in the Enthuware forums, in which that exact question is being discussed and you can ask any doubt you may have about for example one of the wrong answers, or why is that option the right answer. I found that very useful.