The Necronomicon is not actually a Lovecraft work; it's a fictional book that exists within the Lovecraftian universe (and the universes of several other writers, since he did a fair amount of insertion when he ghostwrote for other authors, and it's become distressingly over-borrowed lately by people who only seem to know it from Army of Darkness). The in-print versions you find sometimes are crap.
As Clarion says, you really need to do a lot of Lovecraft-reading. I recommend the Annotated Lovecraft series; it's fairly comprehensive and has lots of interesting footnotes for better understanding. Unfortunately, a substantial amount of "HP Lovecraft" fiction is actually written by August Derleth, under Lovecraft's name but after Lovecraft's death; Derleth's contribution seems to be a highly bastardized interpretation of the original pantheon, to bring it more in line with his personal beliefs.
Pre-Derleth, the Old Ones in general are largely uncaring about humanity; they destroy sanity and kill people mostly by accident, because we're insignificant to them, the way we carelessly kill bugs and such. We try to reach out to touch them and are crushed because our minds are too feeble to understand. Post-Derleth, there seems to be this big good-vs-evil thing going, and humanity has a much bigger role.
So, y'know... do your research! Try to figure out exactly where you want to stand between Lovecraft and Derleth, and find the stories that fit that best. Read and enjoy.
"He wondered at the vast conceit of those who had babbled of the malignant Ancient Ones, as if They could pause from their everlasting dreams to wreak a wrath upon mankind. As well, he thought, might a mammoth pause to visit frantic vengeance on an angleworm." -- H. P. Lovecraft, Through the Gates of the Silver Key
