Oooh, what a great series to have as your formative childhood fiction (and yeah, loving The Grey King the most is super rare — my favourite is Greenwitch)! I came to those books a bit later than you — I was fourteen when I first read them, but they imprinted on my brain in a similar way. The wild weirdness seeping into the domestic (and particularly into the cozy, comforting rituals of childhood) is so beautifully done, and the evocation of the British landscape is just incredible. It's also one of the few Arthurian/mythological-inspired works of fantasy fiction that manages to properly capture the melancholy mood of medieval literature while using a twentieth-century narrative form.