You have to be more specific than that. RF design is a vast subject. If you are interested in an overall basic book go and buy RF design from Chris Bwick.
There is a good book from alan Bensky, this can give you some ideas.
It is called short range wireless communication. Although it is not specifically for rfid, it does cover all the basics.
As cross said RF Design by Chris Bowick is od book. I have one and i can advice u to buy one.
Other good book this area is Introduction to Radio Frequency Desig by Wes Hayward. I also have one and it come with a floppy that contains some RF applications programs.
Do you have the book? I mean e-book.
I found that "Rf microelectronic" from UCLA is better than Lee's "RF IC" because the former one gives you a lot reason. Another one I like is the book RF circuit design with matlab program(try to find @ mathworks.com)