In the PHB under The Book's Appearance the text give two examples of how a spellbook looks like a book and one example with it being just a collection of notes.
Does a Wizard's spellbook need to be a book? Can a spellbook look or even be something else rather than just a book?
For example, could it be a series of small scroll tubes tied together like a rope ladder?
A necklace of marbles where each marble was inscribed with a single spell?
Elaborate wood carvings on a puzzle-box?
Colourful tattoos on the backs of slaves? (I'm not saying it won't be awkward when a Dragon eats pages 21 to 34 when adventuring, just asking if it would be allowed.)
Runes on the faces of convex regular icosahedron? ;)