1979’s The Door into Fire is the first volume of Diane Duane’s Tale of the Five. As I recall, there were to be at least four books in the series, but as of this posting, only two further volumes, The Door into Shadow and The Door into Sunset, have been published. A fourth book, The Door into Starlight, is mentioned on ISFDB, but only as “unpublished.”
In a world where grimdark rules the fantasy bookshelves, this book may seem like an oddly touchy-feely secondary world fantasy. It was even more remarkable back in the Disco Era, when it was first published.