If you like to think about a book as a small, cranky animal, it is kind of fitting that they are often covered in skin. Books can be covered in all sorts of materials—cloth, paper, wood, card—but the oldest and most traditional of these is animal skin.
The most familiar form to most of us is leather, of course, though it’s not used so often these days. Leather is generally made from calf, deer, pig, sheep or goatskin though most animals have been used to make leather at some point or another. (Possibly even squirrels, though rodents are really getting a bit small to be practical).
To make leather, the skin is tanned with a vegetable tannin (eg from certain types of bark), chromium salts or aldehydes (eg formaldehyde). Vegetable tanning was the earliest method used; these skins are good for stamping but are less stable to water—if wetted, they may discolour, stain and become stiff. Chrome tanning was developed in the nineteenth century; a greater variety of colours can be achieved. Aldehyde-tanned leathers are usually soft, white leathers. Brains were even used to tan leather, sometimes! Zombie leather! Eww. The tanning process alters the chemical nature of the skin, so that it resists putrefaction. It allows the skin to be shaped, moulded, pasted and dyed.
Vellum and parchment were also used to cover books. These are also prepared from animal skins. The difference between vellum and parchment is a bit arbitrary—it depends who you talk to. Some define vellum as unsplit calfskin and parchment as split sheepskin. Some say parchment was used for the insides of books and therefore a bit finer than vellum, which is used for bindings. Some people don’t differentiate between them at all. I tend to call it all parchment, even though vellum is a cooler word. To make parchment, the skins are scraped and stretched and limed, but not tanned. Parchment is still a very stable material, really, but don’t get it wet. Or hot. (If you wetted a piece of parchment and tried to iron it dry it would shrink up like a Twistie packet in the oven).
Alum-tawed skins sit somewhere in between parchment and leather proper. Aluminium salts are used to treat the skin. The resulting material is not quite as supple as leather, nor as resistant to water, but they are more resistant than parchment. The ones I’ve encountered pong a bit.
Leatherette is not leather. (Surprise!). Leatherette is made from a coated paper, embossed or printed to look like leather. Do not be fooled.