1)which is the best book available in market for scja exam.
Currently there's only two, which are mine.

I believe Sun is scheduled to release one of their own in June or July of 2008, although that date has slipped from June 2007.
2)is it neceesary to appear for scja before appering scjp?.
SCJA is the new entry level
Java exam from Sun, but it is not a pre-req for SCJP. SCJP is much more syntax and nuance focussed, so you need to know
alot of what might be considered 'obscure' details about the Java language. SCJA is much more 'big picture' where you have to understand the basics and fundamentals of the technology, but really intricate parts of the language and syntax are not required. Still, you need to know how to write basic Java code for SCJA.
3)i have heard abt Cameron McKenzie's book on scja? is these book is solely enough for preparation?
Nope, simply reading the books will not get you certified. Well, they might, but I wouldn't recommend it.
To get certified, you have to write code, not just read a book. So, you have to get your hands a little dirty with the compiler, you have to compile code, write simple loops and conditional logic statements, understand flow control and know basic syntax. You just can't learn that stuff from reading a book alone - you've got to write some code.
But, for the most part, students of mine who have taken a very basic, introductory programming course in Java or C, who then pick up my books, pass the exam with exceptionally high marks.
4)from where i can Cameron McKenzie's book?
The books are available from a variety of European and American booksellers. You can also get them through my website.
5)is these book is available in india. if yes where it is availble in mumbai?
The books can't be found on bookshelves in India, but I ship there regularly. A book that went out from Toronto on a Thursday arrived in Bombay on a Monday. I certainly don't guarantee that type of delivery time, but it was pretty darn kewl.
If you have problems ordering the books, feel free to message me through JavaRanch. I'll help you out!
6)which are other good books availble in market for scja preparation?
I'm a pretty big fan of Head First Java. That's a great book for learning Java, and it will get you into the SCJP topics as well. I think that's a book for someone that understands a bit of programming though.
For a total newbie, I really, really like the Java Progrmaming for Dummies. Not Java for Dummies, but Beginning Programming with Java For Dummies (For Dummies (Computer/Tech)) by Barry Burd.
The thing is, these books will teach you the syntax part, but won't tell you what you need to know about
J2EE, EJBs,
Servlets, JSPs, UML and OOA & OOD. Those are much biggers aspects of the exam than just syntax. I think you'll find that my two books cover those topics rather thoroughly.
By the way, thanks for letting me talk about my books.
-Cameron McKenzie