Grant knew Crowley, he worked briefly as his secretary in 1945, at the very end of Crowley's life, and he knew Spare well, and his own work is largely an attempt to fuse the two and push the result somewhere neither had gone.
The Typhonian trilogies, nine volumes from The Magical Revival (1972) onward, develop a magic of the nocturnal, lunar and reverse side of the Thelemic cosmos, drawing on tantra, on Spare's sigils, on the Tunnels of Set, and, distinctively, on the fiction of H. P. Lovecraft, which Grant treated as unconscious contact with real extraterrestrial or extra-dimensional intelligences rather than as invention.
It should be clear what kind of writing this is. Grant's books present etymologies, cross-cultural identifications and historical derivations that do not survive checking, and his method is associative rather than evidential; scholars who have examined the corpus treat it as visionary literature that uses the grammar of scholarship. Readers who take his historical claims at face value will acquire a great deal of false information about Egyptology, Sanskrit and the history of religions. Read as what it is, a sustained imaginative construction, and an influential one, it is a different and more interesting proposition.
His authority claims were also disputed in his lifetime: he was expelled from the Ordo Templi Orientis by Karl Germer in 1955 after issuing a manifesto of his own, and continued regardless, running his own Typhonian order for the next half-century.