It's been two decades since we last saw Thomas Covenant, a modern-day leper traveling between our world and a parallel realm of magic, creatures, and arcane language. Now the hero of Stephen R. Donaldson's landmark fantasy saga returns in The Runes of the Earth, the first of four books that will complete the Covenant series (whose first six titles have sold 10 million copies). Why the delay? Donaldson, 57, says that he just wasn't good enough to finish the story at the time. ''I had an encounter with my own limits.... I needed to stretch and grow as a writer before I tackled this final installment, so I turned to other storytelling ideas,'' he says. ''One thing led to another and suddenly it was 21 years later.'' In the meantime, Donaldson wrote 10 other books and enjoyed his life in New Mexico though he'd rather not say exactly where, a lesson learned after a zealous young reader dropped by while the author was writing the second trilogy 25 years ago. ''This kid hitchhiked from Iowa with his backpack and arrived at my door, assuming that he could live with me while I taught him to be...me, '' he recalls. ''I practically had to drive him away from my doorstep.'' With Covenant back, Donaldson might have to lock his doors again.