Wow! Most tomes are only good for investigators whose names rhyme with Crazy Stalker -- but this might be the rare exception. Assuming I read this right, you peruse the top nine cards of the encounter deck, choose one, and then discard a copy of that card that is currently in play. In other words, if there's a conglomeration of spheres squelching around your map, and you find his twin brother in your 9-card draw, you can discard said conglomeration. Or if someone has a hex or a miserable Frozen in Fear in their threat area, and you find the match, you can discard that too.
This is impressive versatility! Each time you crack your codex, you have a decent chance of discarding literally any non-elite encounter card currently befouling your board -- whether it's an enemy or some treachery, whether it's at a location, in your threat area, or attached to the act or agenda deck.
Let's think about this. We have some cards that let us discard enemies, like the Disc of Itzamna, or Persuasion. We have a couple that can cleanse your threat area -- Logical Reasoning and the very fancy Cheat Death, which you'd only use in an extreme situation. I don't know of any player card that can pick off cards attached to the agenda deck. And here comes this codex that can do all three! All three three times!
Of course, the the codex does come with some chanciness. You may not find the duplicate you were hoping for in your 9-card draw. If there are only a couple encounter cards on the board, you might whiff altogether. The codex gets better and better as the encounter deck dwindles, and as encounter cards start to pile up on the board. It depends on the scenario, but it seems to me that in a three-player game, it doesn't take long for the board to start filling up, and thus for the codex to become reliably useful. If you play it with 3 or 4 encounter cards on the board and 20-25 in the deck, you'll probably get to discard something, even if not your first choice.
So who should take it? In my view, anyone who can... As with all tomes, Daisy can use the action for free -- always nice. Mandy can potentially discard TWO cards from play, which sounds amazing. But really, I think it's great for any investigator. Joe and Roland might prefer to keep their hands free for weapons, but sheesh, the codex can be a pretty dang good weapon in its own right!
If you want to make the Codex a key part of your deck, consider including means of extending its usefulness as well. You might take Astounding Revelation, which will let you add secrets to it, or Knowledge is Power, which will let you use the Codex without burning a secret. For an item this good, it's worth eking as many applications from it as you can!