Here is a list of The Fionavar Tapestry Books in Order.
#1 – The Summer Tree (1984)
From the blurb:
It begins with a chance meeting that introduces the five to a man who will change their lives: a mage who brings them to the first of all worlds, Fionavar.
In this land of gods and myth, each of them is forced to discover what they are and what they are willing to do, as Fionavar stands on the brink of a terrifying war against a dark, vengeful god…