Just a warning: at least for me, once I got started, it became addictive to try, Oulipo-like (http://en.wikipedia.org/wiki/Oulipo), to write simple programmes in these ever-more-restrictive languages. I was helped along immensely by Hindley–Seldin (http://www.cambridge.org/us/academic/subjects/computer-scien...) and, for, as one would expect, a more friendly but still thoroughly mathematical introduction, Smullyan (http://en.wikipedia.org/wiki/To_Mock_a_Mockingbird). I am not alone in my admiration for the latter; see, for example, http://www.angelfire.com/tx4/cus/combinator/birds.html.

EDIT to add: Speaking of 'Mockingbird' articles, you may be interested in Appendix A of http://dkeenan.com/Lambda/index.htm, whose author discusses another interpretation, attributed there to Barendregt, of Booleans and numerals. For someone who regards the Church encoding as 'intuitive', they were very surprising!