I must say, as one of those that have answered, that I only stated my opinions on this situation and what I would do. Not that my opinion is superior thatn the GMs, or that he knows what is fun for the player, just what I would do if I was presented with that situation at my game table and if I didn't want to share these information.
I do like to keep secrets from my players, but I do not stop them from reading books. If they read the books they do so on their own intent - even though I do tell them that reading the monster manuals might spoil the suspense. Suspense can be one of the ways to achieve a fun game, but it is not all!
I know one instance where I knew every player around the table had read a specific monster manual entry. However I still could use said monster to create an angsty feeling because they all didn't knew it was that monster for I didn't named it at all and to them it was just 'a crazy monster-blanket that fell from above the door' - until I eventually told them at the end of the module that it was the classic Executioner's Hood. They did recognize the mimic they met later, but that is hardly a feat as the mimic is iconic.
However, the same situation did play out pretty different in an other group: I knew only one might have known the entry at all, for he was GM in an other group. I named the monster as an Executioner's Hood from the beginning, and it made one of the player's eyes go wide with terror - and it wasn't said player that GMed elsewhere! I didn't knew this player (who was playing a d20 game the first time) had read some kind of blog entry about the "most silly monsters in D&D", and it was starting to dawn upon him that the name I had given the module (House of Living Things) was meant like it was written. For the whole evening he started to become paranoid about entering any room as he said he feared to meet a Lurker or Trapper. He said it was all fun for him in the end, as it kept him on the edge all the time. Still, him knowing about the fact that there was a whole array of beings that played objects to eat their prey created a fully different type of suspense than the players just not recognizing the monster as what it was.
What I want to say is... that Knowing and not realizing it is something or knowing and realizing or entirely not knowing all depends on the players and the adventure, even the game in question. In Cthulhu or a horror module, not knowing is (imho) the way to go, but other games live of being informed. And then again, there are groups and players that can do the split between meta/player-knowledge and character-knowledge and those that can not.
To me, trying to find a common point across all systems and groups seems impossible.