Now that I've made one pull request I've got the cookbook on my mind, and a few smaller items occur to me, but I wanted input first.

While the point of the cookbook is explicitly to avoid discussing the Deep Magic, there are a few sections where I think a little more guidance on why you might choose something could be helpful.

1. What are the use cases for let over defun — encapsulation across a few functions to avoid globals, generators. Just demonstrating a lexical closure might not say enough.
2. What are other uses cases for values? CL itself offers clues: round, truncate, etc.; distinguishing a failed lookup from a nil value.
3. If you're doing a lot of setting and getting on structs, symbol-macrolet is probably your friend (which is what with-slots expands to for classes).

I assume it makes most sense to do each of these separately, do deal with feedback.