Mabinogion Tetralogy, by Evangline Walton. Overlook. 2003.
This is an under-appreciated work of fantasy. It’s based on the Mabinogion, a set of Celtic myths found in Welsh documents. The version I bought 25 years ago was in four separate volumes, but I’ve linked to a recent version with everything in one volume.
The Prince of Annwn tells of a prince who helps a mysterious Gray Man. A few years later, the druids blame him for troubles in the land, and he vows to take a wife. (Throughout these stories, you see interplay between old tribes and new tribes, men and women.)
The Children of Llyr starts with a time when people are starting to figure out how men are involved in procreation. Inheritance had passed down to one’s sister’s children, since you knew they were related. This is a story of brothers and the troubles between them that resulted in death and change.
The Song of Rhiannon ties the first two stories together, following the story of one of Llyr’s children.
Finally, The Island of the Mighty tells of a sorceress who tried to control someone’s destiny but found she could not.
If you enjoy fantasy, or would like to sample it without picking up something as intense as Tolkien, these are an excellent read.