How much did H.P. Lovecraft actually know about real-world occult literature and practices?

In this 30-minute episode of the YouTube channel, Esoterica, host Dr. Justin Sledge tries to tease out the various roots of H.P. Lovecraft's fictional mythos. Lovecraft was fond of quoting from sources and dropping the names of books, some real, some fictional (his and others). But what did he actual know of occult literature, traditions, and practices?

Thumbnail image: Necronomicon prop, Shubi, Public Domain