Arthurian Legend really does trump quite a lot in terms of interest and subject matter, and the Mary Stewart books are great...
but my favorite read in that vein comes from a series I discovered in junior high (this was before indoor toilets and electricity, you understand)... Susan Cooper wrote the beautifully engineered series collectively called The Dark is Rising. It's important to read them in order, of course.
Book 1: Over Sea, Under Stone
Book 2: The Dark is Rising
Book 3: Greenwitch
Book 4: The Grey King
Book 5: Silver on the Tree
Beyond that, if you haven't thrashed the collective works of the original "Inklings" Oxford literary club, you should:
C.S. Lewis (The Chronicles of Narnia): Despite a substantially better and more serious approach by the films, nothing ever beats the books, though the nostalgia of the old BBC productions (Tom Baker as Puddleglum = /win) is considerable.
Lloyd Alexander (The Prydain Chronicles): Another very easy-to-read-engineered-for-children and yet timeless tale spanning five books if I remember aright.
J.R.R. Tolkien: Perhaps the most gifted linguist of the 20th century (though Stephen Fry is in close competition here): Again, one must ignore the travesty of Hollywood production and return to the glory of the printed word. Read that series all the way through, starting with The Silmarillion, then The Lays of Beleriand, following with The Hobbit (always fun to download the 1973 animated version for a lark) and going on to the Lord of the Rings series. Sir Gawain and the Green Knight, Farmer Giles of Ham, and The Isle of Lost Play are all veritable works of genius besides.
David Eddings has a few full series of books that are quite good at occupying the mind, and a marvelously constructed universe, even if his characters from series to series all tend to resemble one another. (The Belgariad) is your starting place, and possibly some of his best hallucinogen-inspired work. (The Malloreon) is the follow-up series, [these two are five books each] and I would heartily recommend if you like these to progress with his other work: (The Elenium) is as I say a very close resemblance in character cast to the other two, and yet shows his maturity as a writer in many ways. (The Tamuli) is the other series after that one.
Anne Rice has been oft-maligned as the progenitor of a bad movie here and there, but again the books are priceless in their own regard. The Vampire Lestat is one of the greatest stories ever committed to print. The Body Thief and Memnoch the Devil are good reads, but suspend any notion or echo of the movies for any chance at enjoyment.
Finally and perhaps most importantly, it's time to re-visit Douglas Adams. The Hitch-Hiker's Guide to the Galaxy (again poorly translated at best in the BBC production or the recent movie) is an amazing work of literary prowess. Dirk Gently's Holistic Detective Agency and The Long Dark Tea-Time of the Soul are critically important reads, but I highly recommend proceeding with his non-fiction as well, in The Salmon of Doubt and The Meaning of Liff. A truly inspired genius of a man, and a terrible loss to humanity when he died. Please, my fellow thinkers and readers, eat right, work out, and if you smoke, do it electronically.
That's it for now- Happy reading, wherever your eyes find recognizable symbols!