6th Edition - Call of Cthulhu
Sure it is the old version, but until your current campaign wraps up (horribly of course), you need to have the reference material for your game. We are sold out of the rulebook, but we are still selliing the PDF here.
Go to our shop to purchase it.
And some of the free adventures we have are also for 6th Edition.
Whether you want all the play aids in one combined PDF, or just need the Monster Master to manager the horrors your investigators will face, you can download them here.