This was a book read in the late 80s which presented the unicorn as history instead of myth. The drawings were in brown and white and done to look as though they were very old. The basis of the book was the unicorn as an angelic creature, and one of the stories described it fighting demons disguised as dragons. I’ve already checked the books of Bruce Coville and Jane Yolen – nothing.
I’m almost certain you’re describing DE HISTORIA ET VERITATE UNICORNIS or “On the History and Truth of the Unicorn” by Michael Green. The book does come across as a replica of a medieval text, complete with many detailed drawings of various types of unicorns, their history, their duties in this world, their rivalry with dragons. Supposedly unicorns are creatures from Heaven, sent to protect children and enlighten people. It’s well worth the search if you find it.
Hope this helps!
That book was amazing. Thank you so much!