"The Door Into Fire" is an excellent opening volume in this series. Diane Duane creates a Fantasy world in which a cosmic pattern of multiple overlapping worlds conveys richness of texture portraying a complex interaction of history, myth, custom and spirituality. The characters have a deeper inner life than one normally finds in swords and sorcery novels. Particularly interesting is Sunspark who is genuinely believable as well as genuinely alien!
I would unhesitatingly recommend this very satisfying Fantasy.