I was recently watching a show that mentioned the Treasures of the British Library. Some of my favorite works in their collection are the illuminated texts. Books come up pretty frequently in my games in the form of magic tomes, black market ledgers, and royal genealogical records. As important as the contents are, rarely does my group pay any attention to the physical object. In a medieval setting I imagine books are pretty rare due to the lack of printing presses and literacy, which should make them relatively expensive to start with. Any important book would probably have a cover made from exotic material (griffon hide or dragon scale), gilt pages, and written with exotic ink. On top of all that, if the book had special significance, it should probably be illuminated too.
Between the time need to write and illustrate a book plus the raw materials, a book is going to be pretty valuable without regards to its content. My first suggestion is for DMs to throw books in with scepters, jewelery, paintings, and statues as valuable “works of art” which are portable objects of wealth. My suggestion to the players is: even though craft has been done away with, using whatever attributes characters have in your game to make things, why not consider illumination as an artistic skill. This works nicely into the background of any divine-powered character, especially those trained in a monastery. It may not be as fun a craft as brewing is when you’re hanging out at the Dwarven hold, but its something that might get you noticed by a noble or church leader.
So far I’ve been talking about mundane drawings (although with fancy, expensive pigments). But in a fantasy world, there’s no reason why Illuminations can’t be magical. The act of drawing patterns, symbols, runes, or pictures in a book can be like applying a spell; in fact, this can be the implementation of how books get enchanted. Spells can be placed on mundane books to ward against fire, mites, and evil intentions. Other spells can make the text either legible or illegible to the reader, make the book invisible to thieves, change its appearance to keep up with the times, or let its owner know where it is at all times. Of course, the contents of the book itself can be magical; examples include a kingdom’s chronicles that updates itself, a biography that changes with its owner, or just a regular book of rituals.
The drawings themselves don’t have to be plain. Characters can get up and move around (like in Harry Potter); instead of just one snapshot of a battle or religious event, the illumination can be animated. A book can also be a scrying device that shows the target in vain similar to a crystal ball or silver bowl. It would be cool if it shown scenes were shown in a medieval stylization instead of appearing like a TV show.
The illuminations in a book can be a reward for an adventure, or the macguffin that gets the story going. Players should feel free to add depth to the world by inquiring about a book’s art, or describing the art in books owned by the characters. In particular pay attention to a character’s literacy, and just because he can read doesn’t mean he likes to.