Sometimes, even though they know better, there are certain items which are irresistible. Some PC is going to pick it up and look at it. A spellbook is one such item. This makes it a perfect opportunity to trap the item, but in a way which is a little more subtle than something like exploding runes or poison.
Mispellbook (CR 5)
A old, ornate tome sits closed on the table.
Type: magical; Perception DC 29; Disable Device 29
Trigger touch; Reset None
This book appears in all respects to be a spellbook. However, if anything is read, the reader gets dyslexia. (DC 16 Will Save) When preparing spells from a spellbook or reading spells from a scroll, the PC must make a Concentration check for each spell or it won't be prepared, but will still occupy a slot. The PC will think it is prepared, and will only find out when it is cast, at which point nothing will happen.
This one is nice, because the PC's may not necessarily know that the spellbook was the cause of the dyslexia, meaning that you might get to use this one a second time! Or, even better, just trap one spellbook in a pile of spellbooks and magical tomes. Then, sit back and watch them try to figure out which ones are bad. All the while, there can be some choice, juicy spells in a perfectly fine spellbook that they don't get to use right away.