Oh--Tombs of Atuan and Tehanu, of course. Plus I think a couple of Le Guin's Gifts, Powers, Voices trilogy count. Neveryona by Delany (specifically out of the Neveryon series). Dreamhunter by Elizabeth Knox (and I think Dreamquake too). Charles de Lint's The Riddle of the Wren. Gail Carson Levine's Ella Enchanted and The Two Princesses of Bamarre.

Diana Wynne Jones's Year of the Griffin also, which has a group of main characters but for my money Elda is the protagonist.

Also the Claidi Journals by Tanith Lee and A School for Sorcery by E. Rose Sabin.

(I'm not filtering for quality, here, just quantity.)

Travel Light by Naomi Mitchison is not set in a secondary world but is so comprehensively in the epic fantasy conversation that I am going to say it anyway.