That depends rather on your background. I found Schönfinkel's original paper to be very accessible. Perhaps you will too, if you know a little old-fashioned logic.
I took your advice, and you're right, it is very readable. Seeing it in its original context also helps me understand why a logician would want to invent it.
I read Lambda-Calculus and Combinators:
An Introduction by J Roger Hindley as an undergraduate and found it was very accessible.
I would highly recommend To Mock a Mockingbird by Raymond Smullyan. It will also introduce you (if you are not already aware) to the wonderful world of Smullyan's books.