Nightmares 1 is a collection of monsters for 5e by Cthulhu-famous Petersen Games. It gets a thumb up from this reviewer.
A review of Dark Worlds: The Ritual. This set of adventures brings the cosmic horror of the Cthulhu mythos to fantasy heroes.
The Devil's City: Horror Fiction & 5e Gaming Conjoined is a project to fund the full-length RPG campaign sourcebook Horror in the Windy City which expands on Whispers in the Dark.
The emphasis of this book is helping you use 5e rules to create those Lovecraftian stories and testing the modified system with the adventure. It's about the atmosphere.