I agree with you that all that stuff is off topic in a textbook about Lisp. Some off-topic material is necessary in order to connect the programming topic with the real world. Too much of it will just distract from the focus and add bulk to the page count.
That Fibonacci has a closed form it's completely irrelevant to teaching recursion as a programming technique. It could be mentioned in a small footnote giving some external reference. More relevant is the fact that the naive Fibonacci is terribly inefficient and can be vastly sped up by memoization. Even that is a problem that's not specific to the language and how to use recursion in that language. It has to do with using recursion well in any language, that belongs in an advanced chapter in a book which is not mainly about Lisp but about learning programming using Lisp.
Lisp already has a reputation for being scary, which is unfounded, but there it is. A Lisp book which goes into numerous mathematical rabbit holes will probably just contribute to that meme and have a discouraging effect.
That Fibonacci has a closed form it's completely irrelevant to teaching recursion as a programming technique. It could be mentioned in a small footnote giving some external reference. More relevant is the fact that the naive Fibonacci is terribly inefficient and can be vastly sped up by memoization. Even that is a problem that's not specific to the language and how to use recursion in that language. It has to do with using recursion well in any language, that belongs in an advanced chapter in a book which is not mainly about Lisp but about learning programming using Lisp.
Lisp already has a reputation for being scary, which is unfounded, but there it is. A Lisp book which goes into numerous mathematical rabbit holes will probably just contribute to that meme and have a discouraging effect.