I don't know if this is worth reading, but it is somewhat informative about Welsh myths and what bits of it Lloyd Alexander cobbled together to make the characters, places and things in the Pyrdain Chronicles. I didn't have much of a problem keeping the dramatis personae straight, but it might be useful to have around while you're reading said chronicles in case you think someone or something has arisen earlier, although each entry includes an outline of the actions involving this person, place or thing in the books, so it's not advised if you don't like spoilers.
Like it says in the subtitle, this is a reference work. It is therefore of interested primarily to completists (hello) and researchers, and is laid out in dictionary style. There is little here that you wouldn't (i.e., didn't already) get from the source works, aside from the notes regarding Alexander's sources—chiefly the Mabinogion as translated by Lady Charlotte Guest—and pronunciation guides to the not-quite-Welsh names. Perhaps a few of the longer entries on principle characters may give some big-picture, story-arc insight that might not come as readily from the series as from a concise summary, but that is likely about it. I read it like a narrative anyway, because I am a fan-boy and a goober like that, spread over a period of almost eight years. This is what I am doing with my life?
More information than I though possible about one of my top 5 of all time books 'The Prydain Chronicles". A must read for any fan, info found nowhere else. Looking back I think this set of five books have influenced me more than any other to write about the Welch Mabinogion tails