I documented and released ZPB-EXIF, version 1.0, a pure Common Lisp library for reading Exif data from a JPEG. It is asdf-installable. It is available under a BSD-style license. This paste shows some sample output from different people. See also the screenshot in this journal entry.
I'd like to add Exif writing next, but that'll have to wait a bit.
This is a bit of a johnny-come-lately to the Lisp Exif software market; Kevin Layer of Franz has a utility for it also. ZPB-EXIF does a little bit more than his exifinfo.cl, though, and takes a slightly different approach, so I decided to make it publicly available.