The Box of Delights by John Masefield was perhaps the original inspiration. I thought the somewhat archaic language might be a barrier for modern children, so set out to write a rip-roaring adventure in the same style, firmly rooted in mythology, and Illumination is the result.
Illumination is also informed by the work of Alan Garner, who writes magnificent tales based on British mythology. The Weirdstone of Brisingamen is perhaps the best known, and its sequel, The Moon of Gomrath, set in Cheshire. You might also enjoy Elidor. For older children, The Owl Service is renowned as one of the greats of the genre, and is particularly relevant as it references one of the tales of The Mabinogion, the story of Blodeuedd.
The other major influence is Susan Cooper, particularly her five-book tale of the Old Ones, called The Dark is Rising Sequence. The first story is set in Cornwall, in a village inspired by Mevagissey, not far from where I live. It’s called Over Sea, Under Stone. The second, and most famous, is The Dark Is Rising, and is set in Berkshire; I used to drive past Huntercombe Lane South (where the hero of the book lives) and across the wide common land where the book’s climax takes place, every day when I worked at LEGOLAND Windsor. The third is set back in Cornwall again, and is called Greenwitch. The fourth is set near where my grandmother lived in North Wales, near Cadair Idris, The Grey King. The last book is called Silver on the Tree and all five are recommended.
For some Welsh folk tales, try Gwyn Jones’ versions for children from Oxford University Press. It’s called Stories from Wales.
In terms of books referenced in Illumination, the best translation of The Mabinogion is that by Sioned Davies of Cardiff University. If you’d like to read Gawain and the Green Knight (which was originally written in Middle English), the easiest to read is the interpretation by poet Simon Armitage. Both are erudite, witty and down-to-earth translations for a modern audience.