Fortunata Audaces Adiuvat

LEATHER

One of the less expensive ways to decorate your book was to cover the spine and front and back wood boards with leather. To do this, you simply added paste to the outside of your book, stretched the leather over it, and then pasted it down on the inside of the cover. To hide the edges, you can also paste a piece of leather or wood or whatever works.

Some books are more plain while some get creative with how this is done. For example here's a book with a simple leather covering:

Stamped or engraved leather was also pretty popular and could be further decorated with paint or gold leaf.

Bookbinders and artisans could also make raised patterns by sculpting the desired art out of cord, wood, or whatever else and attaching it to the wood board cover. Then the leather could be stretched and pasted over the raised pattern and tools could be used to push the leather around the underlying patterns, making sure the details don't get lost.

Here's someone's modern reproduction that shows you what it might look like under the leather:

I should also mention that leather is an attractive option because it does a better job at protecting your book, especially against moisture.

CLOTH

Cloth was an uncommon choice for book cover decoration until around the Renaissance, but technically still an option. We don't have many surviving examples becaise fabric doesn't last as long, so it's hard to say for certain how prevalent they were. If you had access to silk or velvet, you could cover your book with that, or you could embroider the fabric for some extra oomph.

The example below is called a "chemise binding" which was most commonly used for books of hours or other personal devotional books.

METAL AND PRECIOUS STONES

"Treasure binding" is a general term for books with ivory, gold/silver, and/or gems on their covers, and they look super fancy! These were usually reserved for special books like Bibles, but I guess if you have enough money, you can shell out enough to bind your fanfiction in silver.

Treasure bindings were often made in addition to leather bindings and fixed to the board with tacks. Because they are made with valuable materials, they were susceptible to destruction - thieves or simply monasteries short on cash could pry out gemstones or rip off the whole thing to sell.

STRAPS AND CLASPS

To keep your manuscript closed (to protect the pages!) it wasn't uncommon to add straps or clasps. They came in all shapes and sizes, so you can choose which one you like best!