Player Reading Monster Manuals. Go give it a read, I'll wait. And if you have a second answer his poll on the side. I'll wait.
There is no way you can enforce the players not to read the monster manuals. If you play, most people I know get the core books. And in a game where monsters are a regular occurrence I believe players would have some knowledge of them through stories, trainers or whatever. Most people who live in my campaign world know trolls regenerate. I'm not going to deny my players that knowledge. Some may argue the more unique monsters the players would have no knowledge. Fair enough.
But this is when a GM needs to take advantage of the player's knowledge and turn it on them. So for shits and giggles let's use a medusa as an example. Stone gaze, and because of Clash of the Titans she may be kick ass with a bow, poison snake head, all of us can recite the list of fine qualities this lovely lady possesses. So players are armed with what they think they will happen. Ah oh, lookie here, this medusa doesn't turn anyone to stone. This one turns them into water, trickle into the river next her layer and wash into the sea. Those stone to flesh potions might as well be orc turds for as useful as they will be. Maybe her gaze is not the catalyst, maybe she breathes out a gas and those who are in the area of effect become nothing more than a puddle.
GMs cannot be lazy. They must put the effort into creating new monsters or at least variations of a critter to build tension when a monster is encountered. Tweaking old monsters, playing on expectations is a great way to startle the players out of their comfort zone.
The other thing is just to make new monsters. Blogs are dedicated to making monsters. You can't swing a dead displacer beast without hitting a new one. But the best monsters are the ones you make yourself. In this case the players will only know them by their description and not their stats. Like I said before, GMs cannot be lazy, they need to put that effort in. Players know as much as the GMs and sometimes more.
This ain't 1979 when only a couple of kids had the books. This is 2012 and everyone has the books in print, PDF, on their phones and ipads and crap I probably haven't heard of.
But I say go ahead and read your little monster manuals, I got stuff in my head that's gonna make you lose sleep.