Spare was a prodigy as a draughtsman, exhibited at the Royal Academy at seventeen, taken up and then dropped by the London art world, and he spent most of his life in poverty in south London, drawing, and largely outside every occult organisation of his day. He was briefly in Crowley's A A and did not stay.
His contribution is a technique, and it is one of the very few genuine inventions in modern magic. In The Book of Pleasure (1913) he set out the sigil: a statement of desire written out, stripped of repeated letters, and compressed into a single glyph, which is then charged in a state of exhaustion or ecstasy and deliberately forgotten. The theory is that conscious desire obstructs itself, and that the wish must be handed to what he called the subconscious in a form the conscious mind can no longer read.
What makes this historically important is that it needs no hierarchy, no temple, no lineage, no Hebrew and no gods, exactly the properties that made it the foundation of chaos magic in the 1970s and after, and the reason Spare is now more practised than any of the ceremonialists who ignored him. His wider personal system, the alphabet of desire and the entity he called Zos and Kia, is much harder to reconstruct and is described in prose that resists paraphrase; anyone claiming a clean exposition of it is supplying more order than the sources contain.