Al would be "others", and appears in lower case because it is not a proper noun.
Strictly speaking, al. is actually an abbreviation that could mean any of the Latin words alii, aliae or alia. Which one is appropriate would depend on the gender(s) of said others.
Ah, of course, thank you. I didn't see it anywhere on the review page or the main Amazon page for the book, so it threw me off. I was afraid I was going to have to finish SICP before I'd get this reference. :)
Strictly speaking, al. is actually an abbreviation that could mean any of the Latin words alii, aliae or alia. Which one is appropriate would depend on the gender(s) of said others.