Hello Ganesan Ponnusamy, many people who pass java 8 OCA and prepare to OCJP 8 and people who planed upgrade to OCJP 8 inquire what study material to use?
I also inquire what book you recomend to use? And how to organize study process most effectively?
I cannot find any exclusive book for OCPJP 8 Exams (1Z0-809 & 1Z0-810), however you can find general Java 8 programming books.
You can find free tutorials from Oracle's site to start learning for OCPJP 8 exams until we get formal book for OCPJP 8.
Regarding the study process, I always believe in coding instead of plain paper reading. So do sample programs for each and every exam topics and fine tune your programs with all possible Java 8 options. Later you can attend mock exams to get ready for your actual exam.