good book on combinatory logic

Does anybody know a simple book on combinatory logic?

I read Smullyans "how to mock a mockingbird" is interesting but is a
bit to much about combinators and not a lot about where it is good for
or how you can use it.

Or is the subject matter just to difficult for an simple book?

Am primarily interested how to "simulate" First order logic (predicate
logic with relations / FOL) in this logic.

(smullyan at the end of his book goes into arithmetic but i am loking
for FOL)



Thanks
.