Yes. Studying for that exam will get you to understand Java 8 real well! It's not a cert book, but I recommend Java 8 for the Really Impatient. And my book which is the one that comes out in January . On a more serious note, this exam is really hard to pass without a cert study guide. It also takes more than two months to study for. There's a lot of detail. Which means you are still going to be studying when my book prints.
Also note that you are going to need to learn the Java 7 features as well - syntax and libraries. Do you have any experience with Java 7? If not, I recommend the Java 7 New Features Cookbook.
No, there is an upgrade exam 1Z0-813 for those who hold Java 6 or older certifications. This grants the same OCP 8 title as the full exam. Our new OCP 8 book includes a dedicated appendix just for those taking this upgrade exam. Of course, you need to read the rest of the book to prepare as well.