On the flipside, Lovecraft's work has stretched its otherworldly tentacles across pop culture, music, film, video games, and has spawned countless imitators and innovators. It has remained an underground cult and become a worldwide phenomenon. Those who are avid followers of horror may hear his name in certain circles (probably in the form of the adjective, "Lovecraftian").
I could go on and on about Lovecraft's influences, the definition of Lovecraftian fiction, or his numerous alien gods, but I think the uninitiated should explore for themselves. With that in mind, I've comprised a list of the HPL books that got me hooked on the "Cthulhu Mythos":
The Annotated HP Lovecraft is, in my opinion, the best place to start for the uninitiated. There are more comprehensive collections out there, but Lovecraftian scholars S.T. Joshi and Peter Cannon's annotations of Lovecraft's adjective-heavy archaic prose and obscure literary allusions proved invaluable to me as a first-time Lovecraft reader.
More Annotated HP Lovecraft is the companion to the above collection. It contains the same scrupulous research done by Joshi and Cannon with a second set of stories. Again, a great place to start for Lovecraft neophytes.
Necronomicon: The Best Weird Tales of HP Lovecraft is a very comprehensive anthology. I recommend this for those who have had time to digest Lovecraft's archaic writing style. The majority of HPL's best work is here.
The Dunwich Horror and Others is part of the Arkham Press editions that were released during the 1980s. This collection contains the "corrected" versions of Lovecraft's stories. I use this for my critical studies on HPL. This can be a bit pricey, and is recommended for the experienced HPL reader. S.T. Joshi's corrections (or, restorations, if you will) present a notable difference in the stories throughout. Joshi actually recommended these to me when he found out I was using other editions for academic work.
Dagon and Other Macabre Tales is a second volume from Arkham Press. This also contains the corrected/restored versions of HPL's stories.
At the Mountains of Madness is the third Arkham edition, again with the corrected versions of the stories.
This post has been quite praiseworthy of H.P. Lovecraft. But, I'd like to issue a warning for the uninitiated just the same. Despite Lovecraft's enormous influence on horror, his prose is heavy on adjectives (some of HPL's favorites include "eldritch," "daemonic," gibbous," and "cyclopean"), there is little to no dialogue throughout the stories (or, on the flipside, Lovecraft's attempts at it can feel unnatural or just be plain hard to read) and some of his stories contain unapologetic racism and sexism throughout. So, in short, Lovecraft may not be for everyone. However, despite these flaws, I feel H.P. Lovecraft's work overall to be nothing less than a cornerstone of modern horror fiction. So, get out there and explore the dismal cosmic horror that is Lovecraft's universe.
Shameless self promotion: The Lovecraft Annual No 3 (2009), edited by S.T. Joshi, contains an article that I wrote about utopian ideals in Lovecraft's work.