Look at the listings on http://used.addall.com to see which editions were published in which bindings. It was common at that period for an edition to be bound several different ways, maybe a change in binding colour with each print run.

Leather was unusual. I've never seen a full leather set of these books. Be sure what you've got really is leather and not just something that looks like it. You're talking about at least a tenfold difference in market value.

The really odd format for late Victorian stuff was publication in "divisions" rather than volumes. These were often issued to pre-publication subscribers. They issued the books in a fixed page count and just chopped each book off when they hit it, no regard to logical structure. Not sure if that was ever done with the Minstrelsie books but the same publishers used it for other large publications of similar complexity. The inconvenience means they're not specially valuable.