The magic in question operates on a magical language of runes that give off a constant heat. When the runes are "at rest" (as in, nobody is reading them) the heat is very mild but warms up when observed. When a magician reads them aloud to actually cast the spell the heat rapidly escalates. Exactly how hot it gets varies on the complexity & length of the spell but once you get beyond basic beginner spells it becomes very easy for spells to burn through paper.
The world is at the tail-end of an industrial revolution (so >/=1900 tech-wise) and an enterprising magician wants to make a new medium for storing spells. Paper scrolls are light, relatively easy to make, and portable but can't stand the heat & so only good for limited-use disposables. Clay tablets are much more durable & can withstand long-term use but it can get heavy and so isn't convenient for frequent traveling.
They want to make a spell book that can be reasonably portable for individual magicians (either traveling on the road or bustling about their local city), easy enough to make that they could be mass-produced, and durable enough that they don't have to be constantly replaced (emphasis on constantly, as they assume that the frequent heat damage would render the book unusable eventually). What are their best bets for materials, both for the medium spells are written on and what they're written with?