Henk Barendregdt's "Juggling With Combinators" (http://repository.ubn.ru.nl/handle/2066/17290) shows you a lot of interesting things you can do with Combinatory Logic.
Barendregdt's "Reflection and its use" (ftp://ftp.cs.kun.nl/pub/CompMath.Found/biologen.pdf) has a small, but interesting section on Combinatory Logic. The exercizes include finding a CL term that Barendregdt calls an "Ogre", same as Smullyan's Hopelessly Egocentric bird.
Jan Willem Klop's "Cycles in Combinatory Logic" (http://web.mac.com/janwillemklop/Site/Bibliography_files/10....) does a lot of neat things in a proof that is very hard to understand. It has a section at the end about CL bases other than SKI that's very interesting.
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