So long ago I read Raymond Smullyan's delightful To Mock A Mockingbird, a gentle introduction to combinatory logic (representing combinators as 'birds' singing back and forth to each other). I fell in love with the book and was fascinated by the subject, but most of the other information I've found has been much more dry (which isn't surprising) and seems geared primarily towards practical applications (which is, a little bit).
Is there a good next step that explores combinatory logic primarily from a foundational level, particularly one that explores some of the basis and representation issues that Smullyan's book covers? It seems like the topic may to a certain extent be a 'dead subject' in that essentially all the information that could be wrung out of it already has been, but I'd still love to find another work that surveys the subject broadly but from a more technical perspective than Smullyan's book does.