[gdl]

"The Little Schemer" and "The Seasoned Schemer", maybe? They're well-regarded around here and written as a lighthearted back-and-forth dialogue. I haven't gotten around to reading them yet, but quickly skimming through them on a hunch it looks like normal variables don't show up until midway through the second book (and even then are used minimally). The books use lots of small examples to teach functional programming from the ground up, and so use literal data values instead of variables to keep things concise and focused.