Answer:
If by the Cthulhu mythos you mean any story written about Cthulhu, you are probably out of luck. Writers have been adding stories to the mythos since Lovecraft started it, and depending on how you define it, even before (Bierce and others).
If you mean specifically those written by H.P. Lovecraft, there are several. Even a sub-par iPhone app. For theme, I recommend the Necronomicon, the complete works of H.P. Lovecraft. It's on amazon for about 20$.
All of his works are public domain now, so a quick google search for complete works of lovecraft will also grant you access.
Now, if you want to keep reading about the Cthulhu/Lovecraft mythos, it's a loooong road, and 90% of it is pretty rough stuff. But there are some good gems out there. Start with Clark Ashton Smith, Derleth, Zealia Brown Bishop and other contemporaries and work your way outwards, until you hit the new lovecraft circle. Then, if you are truly brave, you can wade into more modern waters.
Happy hunting!