Hello Ranjan Raut,
Welcome to CodeRanch!
As some of the members have already mentioned, those two are same books. Secondly, Sierra & Bates is not a collectible book. Please don't get me wrong, but that is not a standard Java book. It is a very good book purely focused on OCPJP - which means, you'll rarely refer it after you clear the exam. And finally, hardcover book is around 6 times more expensive than paperback. So, paperback is a natural choice.
Now, coming to the other question of yours, yes, for OCPJP7, you have to clear
OCAJP.
So, you have two choices:
1) Do OCAJP, followed by OCPJP 7
2) Do OCPJP 6, followed by OCPJP 7 upgrade exam.
If you don't have any prior experience of SUN/Oracle certifications, I would suggest to take approach 1. OCAJP is little easier than OCPJP, and syllabus is evenly distributed over OCAJP and OCPJP 7.
I hope this helps.