I really like The Stone Book Quartet (which is one volume, containing four interlinked novellas) -- it doesn't encompass fantasy in the same way as The Owl Service but it has the same sense of the numinous and the importance of place in the imagination. Really good descriptions of snow / stone! I'd also recommend The Voice That Thunders, which is totally different -- it's a collection of essays about writing, imagination and depression.

Many of his books are very strange. I'm really torn on Red Shift which I reread recently. It was written directly after The Owl Service, and it pushes the dialogue / leaving things left unsaid aspects of that novel much much further: it's really hard to figure out what the hell is going on, and there is very little explanation given other than dialogue. The structure is interesting, it's allusive, it's gripping. I don't know if I recommend it, but it is worth a look.