The books have all been written in such a way that you should be able to pick up any one and get a full and complete story. However, characters do reoccur, off center stage as it were, and their stories do follow a sequence. The best place to start is the collection Dreams Underfoot. From there they go pretty much in this order:
The Dreaming Place
A Whisper To A Scream (originally credited to "Samuel M. Key")
I'll Be Watching You (originally credited to "Samuel M. Key")
Memory And Dream
The Ivory And The Horn
Someplace To Be Flying
Moonlight And Vines
Forests Of The Heart
The Onion Girl
Seven Wild Sisters (also available in Tapping the Dream Tree)
Tapping the Dream Tree
Spirits in the Wires
The Blue Girl
Make a Joyful Noise (chapbook)
The Hour Before Dawn (collection)
Old Man Crow (chapbook)
Little (Grrl) Lost (novel)
Promises to Keep (short novel)
Dingo (short novel, forthcoming)
The Dreaming Place and The Blue Girl are YA novels. A Whisper To A Scream and I'll Be Watching You are, respectively, a horror novel and a thriller; they're darker fare than the other Newford books and aren't really that integral to the underlying, ongoing backstory that takes place off center stage in so many of the books and stories.
I swear I looked at his website...I really did. And it was inevitable that I would have started halfway through the series. Now I am off to the library website to see which ones I can and can't get hold of.
The other issue is that of course the library doesn't have the whole collections. It is missing book numbers 3 and 4, 6 to 8, 12 and 13, 15, 17 to 20 and 22. Ah well. I'll cross that bridge when I get to it!