Just make all spellbook have some strange effect so you players will not sell the extra's.
Most of the spellbooks NPC wizards have, are research books.
A research book contains the research on a spell and can have some additional strange and wonderfull effects. A wizards research into the Magic missle spell might contain the Magic Missle spell in 3 different versions.
The magic missle spell, the standard spell
The Magic bottle spell, as the magic missle, but the missles look like broken beer bottles, nice for barbrawls
The Magic Missle spell, that has a small chang to bypass the shield spell if special material components are used. the ingredients of these components are written in the same spellbook, and might contain several pages with the recepy.
Further the shield spell will be in the book and more spell like it.
The book will also give you a +2 to learn magic misslle or shield related spell.
Now nobody will sell the book
And of course just get you spellcraft up so much that you can learn from other book.
This is a great way to create pc wizards libraries....