Unfortunately, I don't think there are any books on iconography from a Coptic perspective that have been translated into english. However, I do have a short 50 page pdf document written by His Grace Bishop Daniel (Bishop of Sydney and its Affiliated Regions) entitled "The Spirituality of Icons". This document has reasonably-sized coloured depictions of many of the specific Icons that he discusses as examples depicting essential concepts and themes, but there are no full-size illustrations i'm afraid.
The file size is about 1.8 Mb, so I can't attach it to this post. If you (or anyone else for that matter) would like to PM me your email address, I can send it to your email if you're interested.
as far as I'm aware Coptic Iconography as a discipline has only really developed in the 20th century
Not true. Coptic Iconography has gone through its various stages, like any other form of art. The 20th century merely represents one such stage.
