Before the introduction of modern methods of paper production, most European books were made of wood, metal and leather. They were heavy, bulky and expensive. Spell books are no exception; in fact, if anything, they tend to be even more expensive than their mundane brethren.
The example shown here (a medieval Bohemian manuscript of 1397) is the sort of thing one might expect to find in your average wizard's workroom or backpack. It's a no-nonsense book designed for the road — the loose soft leather flap is called a "chemise" binding, and is supposed to be wrapped around the body of the book to protect the edges of the pages from dust, dirt and the elements. The whole package is held together with straps, which in this case latch on to simple prongs on the front cover, though more elaborate latches and buckles were also commonly used.
It consists of about a hundred leaves, of which the first twenty or so are paper (added at a later date than the main body of the book), and the remainder are vellum, which is a fine-grained unsplit lambskin, kidskin, or calfskin prepared especially for writing on or for binding books. The covers are of leather-bound wood, the vulnerable corners protected by brass plates. The dimensions of the book are roughly 20cm x 30cm (that's just a little narrower than the Hero System rulebook, and a little taller) and about 11cm thick. It weighs just on 6kg.
This would be an example of a "lowest common denominator" spell book. They may be substantially more elaborate than this one, and they may be much larger and much more expensive. Very valuable books would be likely to include cunningly designed latches and locks to keep out prying eyes and fingers, and covers may be made of exotic hides or metals. And, of course when you're dealing with wizards, there's always the likelihood of magical traps to contend with.
A single spell or prayer book like the one shown here can contain up to 1,000 Active Points worth of spells; about 10 Active Points per double-sided page. They tend to be bulky and heavy, and a nuisance to keep dry and safe. They're also valuable, and a prime target for the nimble-fingered, so watch out.