Imagine if the situation were reversed. You're playing a level 3 wizard who knows every spell except magic missile. A level 1 wizard NPC comes up to you and offers to give you magic missile for free if you let him take a look at your spellbook. Why would you say no?
These are not baseball cards of comic books - having a complete set does not increase their value.
I worked hard to get those spells. As an adventurer, I put my life on the line, either to defeat the owners of previous spell books, or just to earn the 3800 gp required to scribe all those spells. While Magic Missile is a good spell, clearly I've been able to survive without it. That one spell is not worth my entire collection. It is an uneven trade. So, go ahead, and look in my book, and take *one* spell. I'll take Magic Missile, and the trade will be even.
I am not a public library - those probably haven't been invented yet. I am more like an independent contractor. If you want access to my resources, you'll have to pay me for the effort I took to assemble those resources.