I've made these Coptic bound books with covered spines every now and then, sometimes with linen spines with linen covers, sometimes with reindeer leather spines with tar paper covers. The leather spines I glue directly onto the sewn signatures, but the linen spines act like any other hollow spine and are attached only to the covers. Both approaches add to the durability of the book as the covered spine prevents the sewing thread from wearing out due to friction.
Generally speaking the bare spine of a Coptic bound book withstands use well enough as long as the book doesn't have a massive number of pages, which obviously translates into a longer time for the book to be in active use (= more wear and tear), and also affects the general stability of the book. For the heavy users, who never leave their notebook/journal/sketchbook home and lug it around in less than gentle conditions, a book with a covered spine could be a better option in the long run. I take care that the cover material doesn't affect the phenomenal ability of a Coptic bound book to open well too much. Naturally adding stuff (i.e. leather) to the spine makes the spine less flexible, but these books still likely open better than any machine made book you can buy from a shop.
While I have all sorts of leather in my stash, I've never felt like making these books with anything but Finnish reindeer leather. It's soft and pliable, and I like the red-brown colour a lot (it's over-dyeable, though). The surface isn't like any regular bookbinding leather, as this has a much softer, completely matte finish, so it's not something I'd use for a full leather binding unless the easy scarring of leather would be a planned feature. For spine-only use this is pretty perfect, though. If you're interested in purchasing some reindeer skins or parchment for your creative endeavours, head over to this site (some in English, some in Finnish only - totally unpaid and unasked advertising here).
Out of these three books only the hourglass book is for sale. V reserved the solar eclipse book before I had even finished it, and the little cyclist (under a layer of mica, in case you were wondering) is a personal journal of mine from some years back - I just completely forgot to share it when I finished it. The hourglass book is 12,8x17,7x2cm and it has 96 pages of recycled grey-brown paper, and you can find it for sale here.
I've been thinking about coming up with new cover materials to pair with this reindeer leather, but I'm drawing a blank. I'm not a huge fan of combining leather with book cloth or linen, and most papers just don't seem to have the durability I'm after. If you have recommendations for new cover materials I should try out, I welcome them with open arms!