There is a book of Arthurian Tales done in large slim paperback, maybe 60 odd A4 pages in all with greyscale pencil illustrations, aimed at a late primary early secondary school audience. It's what I think of as "middling abridged", having the basic stories of the Green Knight and Mordred and most of the others but not covering the full myth cycle nor being done in a traditional saga style. I love the art and have been trying to find it for years, I remember that Mordred is pictured in a smooth faced helm in the final battle scenes and the Green Knight carries a Butterfly Axe but not much else about it.
It's 330 pages instead of 60, and the illustrations don't look like pencil to me, but your description instantly reminded me of Roger Lancelyn Green's King Arthur and His Knights of the Round Table. Portions of the book can be read on Google Books; does this look like the kind of illustrations you remember?