Zandalari Library Card

It should be noted that trading these or buying them does not get you credit. My husband got one today and traded it to me to see if I'd get credit, I did not.

According to the Wowhead data, the probability of getting each book is ~0.25%. Assuming this is correct, it's possible get some rough idea of how many trolls a typical player would have to slay (in total) in order to get all 5 books required for the achievement. The following list shows the percentage of the playerbase that would get this achievement after slaying the given number of trolls:

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• 300 trolls: 0.9%, don't bet on it.
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• 500 trolls: 18%, quite unlikely.
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• 900 trolls: 57%, typical.
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• 1500 trolls: 89%, quite likely.
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• 2500 trolls: 99.0%, very likely.
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• 5000 trolls: 99.998%, almost certainly.
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For the math-oriented people: Let the probability of getting each book be p = 0.25% and assume that only one out of the 5 different kinds of books can be looted at a time. After having looted N trolls, the probability of getting at least one of every book (S) is given by (the proof is left as an exercise for the reader):

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• S = 1 − 5 (1 − p)^N + 10 (1 − 2 p)^N − 10 (1 − 3 p)^N + 5 (1 − 4 p)^N − (1 − 5 p)^N
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To get some numbers, just copy-paste this Wolfram Alpha input:
1-5*(1-p)^N+10*(1-2p)^N-10*(1-3p)^N+5*(1-4p)^N-(1-5p)^N where p = .0025 and N = 300

Note: If you find an error in this analysis please let me know!

TL;DR: Good luck on the achievement (you'll need it)!

I just looted [item=95408] on the Isle of Giants off of a [npc=69925], the same mobs that drop the Zandalari pets, [item=94126], [item=95422], [item=95423] and, [item=95424], and it did count toward the achievement Zandalari Library Card, so they obviously come from more than just the Isle of Thunder.