So yesterday I asked about allowing players to read monster manuals.
First, let me thank the folks that have voted in the poll, and who vote after I post this. And to everyone who commented on yesterday's post. And check out Tim Short's discussion of the same subject and that of Robert Conley as well. I should have linked to them in my original post, thus creating recursive links that would ultimately destroy the internet. At least that's how Captain Kirk and Doctors two through five seemed to do it. Too late now.
Yes, No, or Other?
My answer: Yes.
Or more like, maybe. They are allowed to read the monster books. I don't discourage my players from reading the monster manuals. I don't encourage it, either. It's purely up to them. I neither make it easy for them (I don't hand out monster books, monster stats they didn't discover in play, or monster descriptions) nor difficult for them (I don't hide anything or say they can't read it).
I don't let them use the books during play, but I don't let them use any books except the magic book during play.
I certainly don't stop them, and as was noted in the comments yesterday, you really can't. People buy books and read them. They GM the game themselves or just read the books as a matter of interest or as a game fun bucket list maker ("Holy crap, we have to fight these guys! And them, too!")
So yeah, I know some of them will read the books and I'm fine with that.
Can they use this knowledge in play?
Ah, trickier. I gather the very, very old school method is "Of course you can, player skill is important." If the GM foolishly uses a monster from a pulp adventure you read and you know its weakness, hahah, you've got the edge. If the GM makes something up to specifically counter your knowledge, well, you should have been more careful. Or at least that's how I read that approach. I could be wrong, and it's been decades since I played close that way.
My method is a bit of a mix.
Yes, you can use that knowledge in play. But it can be risky.
If you know something out of game about a monster, you are welcome to try and use it.
But I make up lots of monsters, I convert monsters by feel and by personal taste when I do convert them, and sometimes I use names of monsters for something very different than you've run into before. I pull critters from types of D&D (which are common knowledge amongst my players, generally), Rolemaster (pretty much only I know them), from my own head (only I know them), and from GURPS sources (somewhere in between - since some of them my players have fought before in my other games).
And if I put a mini on the table that you recognize, or use a monster that you know that I know that you know, understand that I know this, too, and have expectations about it. I don't use Poul Anderson trolls and pretend you won't know what they are. If I plunder from games we all played, I expect you to recognize the monster. I'm not so foolish to think wildly available information is secret, or ask players to firewall that knowledge completely from their characters.
So, you can just risk using what you know. But there is no assurance that it is accurate. I might just be trying to trick you - in that same way "use the player's knowledge against them" way that, say, gas spores are designed to do. But I might change them so it's a very, very bad idea to use fire against them - maybe they're explosive, or burn eternally, or the fumes are poisonous.
And there is another risk, too - maybe I have no idea what that mini is supposed to be, or I have a different rule than the game you recognize the monster from, or whatever. Maybe I just bought the mini/used the name/used the picture because it was cool and made up something crazy to use it for. You never know.
However, your character might know.
Since I run GURPS, and GURPS has nice skills like Hidden Lore, I run it this way:
You can roll against the appropriate skill to a) find out what your character knows [great for monsters the players have never heard of] and/or b) find out if what you (the player and character) knows is actually true. AD&D wights drain levels and you need silver or magic to hit them. Is that true in GURPS? Roll and find out. If you fail the roll . . . sorry, you'll have to try your luck. If you succeed, you can falsify inaccurate information ("Total balderdash about silver, it doesn't do jack to them.") You get one piece of information or one "fact" confirmed/disproved for a success, plus one for each point you succeeded by. This nicely encourages high skill over and above the minimum needed to insure success, and it gives extra benefits for taking time/spending money to do research.
So my players know that wights wait for them in the Caves of Chaos. I allowed the cleric to roll his Hidden Lore (Undead) skill. I think he made the roll by 3 points, so I told him four pieces of information that, as far as he knew, were accurate about wights. It wasn't always very deep information ("Their very touch is deadly, and so is touching them.") but sometimes it was specifically helpful ("They are vulnerable to holy water.") A critical success would have been extremely complete information, with a great degree of accuracy.
Sages are a good idea here, too - you can consult sages and get their roll on the subject, which will augment yours (or replace it entirely, if you don't have the appropriate skill). They can do the same thing - tell you new stuff or tell you if the stuff you already know is accurate. Higher ups in the priesthood are great for info on the undead (or at least in the fighting orders), barbarian shamans or druids for info on crazy animals, demonologists for various demons, etc.
So can that legendary one-armed man in the tavern, if he's around. He's got experience. Speaking of experience, in the same campaign, I think it's fair to pretend the next characters heard all about what happened to the last characters, and what worked for them (and what didn't.) So that way player knowledge is preserved and valued - you don't need to pretend this guy doesn't know about the hill troll's vulnerability to frost attacks or roll to see if Volos II knows the things that Volos found out with his rolls. You do - you heard it from the tales of the last group.
In short, it's a mix - player skill matters, and it can help . . . but character skills are critical to finding out ahead of time if the GM mucked with the monster you think it to be. Fits my "worst of old school and new school" approach, I think.
Feel free to keep voting in the poll - I'll talk about the final numbers when it closes in about a week.