I'd start with the Prydain Chronicles, which are simply wonderful. I'd always enjoyed fantasy novels before I read them as a kid (Narnia & such), but they're what really got me hooked on Welsh and Celtic folklore. Alexander's written a couple other series, plus some stand-alones; if you like Prydain, you should defintely hunt down the Westmark trilogy as well: Westmark, The Kestrel, and The Beggar Queen. They're darker, more grown-up-ish, but absolutely wonderful story-telling.I have tried in vain to find his books in the shops here. I will maybe order online. Do I start with the Prydain chronicles?
I also like the Vesper Holly books, which are about a girl-Indiana-Jones (for a younder audience) but are just plain fun. I don't know much about his stand-alones, except that a couple of them have been reprinted recently, and he comes out with something new every couple of years or so.