Mary Stewart's The Crystal Cave is actually the first book of a trilogy. The other two books are The Hollow Hills and The Last Enchantment. They're very evocative and, um, Celtic, if that makes sense. They also tackle the Arthurian legend from the perspective of Merlin, which is an interesting point of view, and makes the story quite fresh. A companion book (The Wicked Day) picks up approximately where the third book leaves off, but approaches the story from Mordred's perspective. I think the first three books are available in an omnibus volume called The Merlin Trilogy.
However, I maintain that you can't do better than finding a good complete edition of the various works of the old master himself, Thomas Malory. But even he didn't originate the tales -- most came from the French writer Chretien de Troyes. There is one version, edited by John Matthews, that has numerous, beautiful illustrations by Anne-Marie Ferguson! But don't go after a Malory unless you're prepared for a sometimes heavy slog -- most editions don't do much to modernize his language, and Malory's approach to character and narrative are distinctly fifteenth century.
