The Mists of Avalon (1982) is a thick book written by Marion Zimmer Bradley. Following the life and times of Morgain, Arthur's sister, the book deftly and beautifully weaves a tedious and unengaging tale, demonstrating what happens when you turn an action-adventure-romance series into a meaningful historical fantasy.
After about 50 pages, I switched from reading to aggressive skimming with no loss of comprehension. After 250 pages, I abandoned the work. I may attempt to complete the books, but I feel no compulsion.