I met Phil Baker some years ago, after the publication of his book. He was surprised when i said, I'd read Spare and got a lot out of him. I think that's the only downside of his book - he doesn't take seriously the idea that Spare had a coherent philosophy for all it might be expressed in a convoluted style. One of the earliest essay on him was Lionel Snell's "Spare Parts" which was first published in the 70s. Lionel says in that that that looking at Spare's second book Earth Inferno, he felt he was trying to articulate an experience he didn't have words for. He's still groping towards that in The Book of Pleasure in my view.
Probably my favourite piece of writing about him is Steffi Grant's memoir at the beginning of Zos Speaks. It's amazing - it really does evoke a vanished world, a crumbling decaying bombed out London. It's wonderful.
And Crowley and Spare - people talk all sorts of shit about them - "they were lovers" etc which is straight up grade A bollocks. They'd met, Spare did a few illustrations for Crowley's journal Equinox IIRC but Spare basically thought he was an insufferable exhibitionist prick. Which to be fair,seems an accurate impression, borne out by most of the Crowley biographies. Spare disses Crowley, and magicians like him, in the second or third chapter of The Book of Pleasure,describing them as "the unemployed dandies of the brothels". I would by happy one day, to have coined such a phenomenally expressed insult.
You can't really overstate how different their magical systems are from each other tbh. Spare didn't so much stray as reject wholesale and wander off and invent another discipline. He puts a big black line through the whole elaborate Victorian edifice of Golden Dawn magic, burns down that temple, pisses in the corner of the vault of the Adepts. Wanders off down the Walworth Road and crafts something a million times more intuitive and immediate in his Brixton backroom. Outstanding.