[ATDrake]

Bargain @ $2.99 each from Sourcebooks (couponable @ Kobo, price good at the usual Canada & US retailers, may or may not be discounted in other regions, etc.):

By Gillian Bradshaw (ISFDB, Wikipedia), #1 & #2 of her Down the Long Wind trilogy of historically-grounded Arthurian fantasy (starts out a little on the YA side), all three novels of which placed on the Locus lists for Best Novel of the Year when they came out. #3 is not on sale, but has previously been offered at a discount last October, so you might have picked it up then:

• The Hawk of May, #1 originally out from Simon & Schuster in 1980
• The Kingdom of Summer, #2 originally out from Simon & Schuster in 1981

By Anne Eliot Crompton (ISFDB), all 3 novels in her untitled otherworldly fantasy Arthurian trilogy (each readable standalone, centred around the relations of one of the heroes to the faerie world, may tilt a little YA-ish, Sourcebooks is marketing these in its Sourcebooks Fire teen imprint, while the originals came out from adult-level fantasy lines):

• Merlin's Harp originally out from Donald I. Fine in 1995 and picked up by Roc in 1997. This was a freebie a long time ago so you might already have it; I've read and mildly liked this one, enough to pick up the others in series when they went on sale the last time around
• Gawain and Lady Green originally out from Donald I. Fine in 1997
• Percival's Angel originally out from Roc in 1999

By Alexander Key (ISFDB, Wikipedia):

Escape to Witch Mountain, vintage 1968 YA sci-fi exploited teens with psychic abilities adventure (Wikipedia) and basis of the eponymous Disney film (Wikipedia)