Could a Cassisian familiar double as a spellbook using its perfect memory. The wizard could have it recite the required study matrial whenever he/she/it wanted to learn a spell.
Spellbooks do not simply contain information, since Wizards cannot pick up any spellbook and memorise from it - it needs to be their own grimoire (or they need to roll a check).
Also, it's never specified if some of the info isn't actually in the form of graphs or diagrams.
A witch could... but she'd probably turn you into a newt... and you would have that whole unable to sink problem too.
And if you really want to appreciate the difference between a book, and an audio-book... google for Gilbert Gottlieb reading Fifty Shades of Grey.
If you're concerned about the longevity of your spell book you can make an extra for the road. It doesn't need every spell in the primary. If it gets destroyed you still have the original.
This was more of a mental exercise than a serious question, but it would make for a decent backup if it worked.
@VRMH: You actually can use another wizard's spell book to prepare spells. It requires a Spellcraft check to do so, so it's not as easy to do, and until you copy the spell into your own book, you must make the check every time you prepare the spell.
@Earl of Essex0: I think by RAW you could not just have the Cassisian 'read' the spell back to you from memory.
However, if you could find a way to allow the familiar to write - perhaps by polymorphing it into a creature with hands - then you could have it write the spell information down for you, and then you could copy it from that into a spellbook.
It wouldn't allow you to use it as a direct back-up spellbook, but it could allow you to recreate it in case the book was lost - and all by RAW.
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Actually...
Though cassisians can assume other forms (that of a child-sized angelic humanoid wearing a proportional helmet matching the cassisians’ true form, a dove, a dog, or a fish), they find it strange and rarely stay in that form for more than a few minutes.
So no polymorph necessary. You ask it to assume a humanoid form, it writes out the spells, then you copy the spells into your new spellbook. You basically have a floating spell hard drive, and I don't see that it violates anything in RAW.
While a Cassisian could have perfect memory of this, I would imagine that: 1) one would still need to spend the necessary gold on components to write these to a spellbook; 2) just because you have perfect memory doesn't mean you have the ability to transcribe it well, or it would take prohibitively long to do so (especially when you are polymorphed into an unfamiliar form and asked to write with with appendages you may not understand); and 3) because of 2, a spellcaster would still need to comprehend the spell in order to use it.
If I was GM, I would rule that it's possible for a Cassisian to rewrite the spellbook for its base costs, but it follows the rules of a 'borrowed' spellbook.
That said, a spellbook isn't meant to be a tool to cripple the spellcaster at the GM's will. If a spellbook gets destroyed, once is an accident; twice is a coincidence; three times is a vendetta. In any case, I like Treantmonk's way of preserving a second spell book way better than a Cassisian.