I'm willing to achieve Oracle Certificate Java Associate(1Z0-850). But I can't search any good book to prepare for the test. So, anyone has some documentations for this, please suggest me.
I'd recommend purchasing Deitel's "Java for Programmers" and learning how to code in Java. While there are a surprising number of questions on the practice exams that deal with general Java issues, learning how to code in Java SE by actually writing code will benefit you far more than simply memorizing questions and answers.
Check Oracle's site for the topics that are covered on the exam (http://education.oracle.com/pls/web_prod-plq-dad/db_pages.getpage?page_id=41&p_exam_id=1Z0_850) and make sure you familiarize yourself what all of the various Java platforms and technologies (J2ME, J2EE, EJB, servlets, JSP, JMS, JNDI, SMTP, JAX-RPC, etc) are used for.