When Leon Abbot inadvertently journeyed to Earth, he was captured by Minerva Paradizo and given the book and a crossbow. When Leon escaped and returned to Hybras, he claimed that he could save all the demons by using the book. They then followed him, and Leon started illegally mesmerizing demons to submit to his authority.
He claimed Lady Heatherington Smythe's Hedgerow was a book used by the humans that told all their secrets and contained human names. Thus, he changed his own name from "N'zall" to "Leon Abbot" (the name of the general in the book) and forced all the other demons to change names as well, telling them that because they needed to build up an army to fight and subjugate the humans, they would all need human names. The crossbow he brought back too, he told them, was the main weapon the humans used and fought with in war. Therefore, the demons needed to train and fight with the crossbow.
Note: This may be an allusion to "La maison des poupees", a story that was published in 2002 on staylace.com. The story is poorly written, slightly disturbing, full of anachronism - and there is a Lady Smythe-Hetherington. It remains a mystery what business Colfer had on a website that advertises corsets... Although, it might be that he came across it while doing a research on historical fiction.