Just as Israel Regardie’s The Golden Dawn is, though it has its shortcomings, a widely-recommended source of raw information concerning late Victorian British occultism, Agrippa’s Three Books of Occult Philosophy is widely considered the encyclopedia of occultism as it was studied in Renaissance Europe. Indeed, it and the Golden Dawn’s system are not unconnected; Francis Barrett catalysed a resurgence of occult interest in Britain in 1801 with his The Magus, or the Celestial Intelligencer, which largely just plagiarised substantial chunks of the Three Books and perhaps a pinch of a different source in order to deliver its information, and then later when the Golden Dawn was formed great chunks of their inner practices were adopted either from that or, less likely, Agrippa directly.
The reason that it’s more likely that the Golden Dawn leadership got their details from Barrett rather than Agrippa is that for a good long time a full English translation of Agrippa wasn’t available, aside from a 17th Century translation riddled with errors. Donald Tyson’s presentation of the Three Books is the product of a laborious process of reconstruction, bringing the full text back into print in English for the first time in ages, repairing errors, and extensively annotating everything (and when I say “extensively” I mean “in many chapters there’s a greater word count in Tyson’s footnotes than in Agrippa’s actual text”).