Like others, I picked up this book after rewatching True Detective Season 1. I went in blind and enjoyed it. Aside from Zapffe's essay The Last Messiah, which Ligotti repeatedly returns to, The Conspiracy Against the Human Race is primarily an exposition of Ligotti's own philosophy. As a horror writer, Ligotti also devotes considerable space to supernatural fiction, the uncanny, and horror as an artistic form. Those sections didn't particularly interest me, but if you enjoy literary horror you will probably get more out of the book than I did.
The most convincing argument for philosophical pessimism in this book for me comes from Zapffe:
