The Necronomicon: Disambiguation
There's the Necronomicon described in the "Evil Dead" movies.
There's the Necronomicon sold in Waldenbooks.
There's the Necronomicon game supplement(s).
There's the Necronomicon collection of Lovecraft stories.
This is not about any of those.
The Necronomicon invented by H. P. Lovecraft (or "Al Azif" or "Kitab Al-Azif") is entirely fictitious and has never existed in the sense described by the author. As has been mentioned in other threads in this forum, it is (within the genre and mythos Lovecraft created) a writing penned by an Arab named Abdul Alhazred while deep in ancient ruins underneath Irem, where he "communed" with ancient spirits that revealed these truths to him. The original was in Arabic, and the most accurate translations were in Greek, and then Latin (although lesser translations in French, German, and even English were made, and are more likely to be found by an investigator). The contents describe Earth's pre-human history, The Great Old Ones, how they were imprisoned on Earth, how to summon a few of them, and so on and so forth. In short, it's the 8th Century's version of "A Brief History of Space and Time."
This information can be found in various places, but a lot of it is put together in the Wikipedia article describing it. Here's a link to it.
As to its role-play aspects.
Your investigators should almost never get a glimpse of nearly any translation of this book, especially the older Greek and Latin translations. If they get a chance to obtain one in the original Arabic (and not translated from that to Greek to Latin to French then back to ancient Arabic), it should come at a cost that is quite high, and that should include a great deal of their sanity. They must go to it; it will not be able to go to them.
But suppose they've got one, somehow. I don't care how. They've got one, and they still have enough marbles left to stay in the game somehow. What then? Well, that's up to you. But consider this aspect of the Elder Gods and Great Old Ones: while they're against each other, neither is evil nor good.
They're amoral. They don't have any concept of good or bad, right or wrong, malevolent or benign. They do what makes them feel happy. If that's eating the planet, then it's what they'll do. Humans are meaningless to them. Oh, sure. Nyarlathotep involves itself with Human affairs once in a while, and so does Hast... uhm... The King In Yellow. But mainly because they're bored.
Furthermore, The Necronomicon is not, in and of itself, inherently evil any more than an untripped land mine is evil. Handled properly, it's safe enough. In the hands of the ignorant, careless, or foolish, it's going to blow up in someone's face. So just having a copy (any copy) doesn't make the players evil. Using it doesn't make the players evil. It's how it is used, and what that user's intent is, that makes its effects evil. Like a bullet from a gun, the book (and the Old Ones) has no concept of whether or not what it is used for will be evil. Perhaps it will release a Lloigor that destroys a village. Perhaps it will break a mystic seal, allowing the rescue of a trapped person. Perhaps it will simply look nice on the shelf, between Cultes Des Goules and Unaussprechlichen Kulten and a couple of skull bookends.
Some notes, then, on how it should be introduced into your campaign:
-It should be very hard to recognize. The title probably won't be on the spine of the book. If it ever was, it's probably long since been worn off. The investigator will need to be able to actually read the language, open the book, and read a page or two... and by that time, it's far too late. If they get a translator, the unwitting fool should scream, run out of the room, and die in a churchyard of apoplexy.
-It should be very hard to obtain, once recognized. Even if the investigators manage to get into the Rare Books section of Miskatonic University's library, they'll need Very Special Permission to see that book, and even then they'll be observed by an attendant. Sure, they can take notes from it. But they can't take any pages from it, or even remove the book. Anyway, the thing's probably so old and fragile that any attempt to move it from a shelf to a table would be an adventure in itself. Removing it from the library would likely destroy it. If they find one deep in some underground cellar, it should be guarded and protected, and even if they get past that defense, the thing will probably fall apart in their hands. Give them one chance to read it or take notes, subract sanity points appropriately, and then let it disintegrate into a mouldering pile of rot.
-It should be very expensive to obtain. Whether it costs the players cash, health, sanity, respect of their peers, freedom, or any combination thereof, it should cost a lot of it. This is the literary Holy Grail of the genre, and just giving it to the investigators would be like giving God's email address to an agnostic. They should go through some serious investigation, spend a lot of time and money, and sacrifice more than they even knew they had just to touch the cover.
-Having acquired it, the rewards should be commensurate to the effort put into finding it. Be fair. Give them every opportunity to obtain the information within it, and record it somehow, before it dries up and blows away. Let them photograph each page if they have the ability. Or let them copy the entire thing, if they have enough time. Perhaps let them carry it out of the place where it is found, let them leaf through it, stressing that as they turn each page, it falls apart. Give them fair warning that they'll only have one chance, and they'd better take full advantage of it. It should boost their knowledge of the mythos more than any other book in your campaign. It should also make them as crazy as a cannon made of jelly. Only the strongest minds should even try to grasp its contents.
-If they do survive with their minds intact, be ready to throw the serious stuff at them. Ghouls? Night Gaunts? Hounds of Tindalos? Pah. They'll be running into Nyarlathotep, Dagon, Ithaqua, and Cthugha now. They'll have the information they need to escape, and maybe even thwart, the designs of these entities. Give them the chance to do so. Keep it interesting.
Interest their brains out. After all; they opened the book. They asked for it.
God is my witness; nobody else saw a damned thing.