The Mary Stewart "Aurthurian" (I've only read 1st 3) are OK, but I prefer most of her other books I've read. The "Moonspinners" is better than the film which was miscast and poorer. I've read loads.
Has Legacy just got the 4th book added?
Amazon claims 5 in the series, though they have Heyer books as series that are not.
The Prince and the Pilgrim
£18.94 for all five as Kindle.
https://www.amazon.co.uk/dp/B00713DNEE