Which Bestiary to Buy?

[Alton_Utrich]

I've got the funds to buy a new Pathfinder Sourcebook and I'm gunning for a new Bestiary. I own the first one and have access to the third one. My next campaign is going to heavily feature demons, devils and undead. Which Bestiary would be best to buy to get the most of those? Are there any other books I could pick up that would be useful for those three?

[Palanan]

If I can chime in with a follow-up question, complementary to the OP: which Bestiary is best for unusual animals, clever magical beasts, and/or modest examples of Far Realms abominations?