The Once and Future King by T.H. White?

The Princess Bride by William Goldman
The Paladin by C.J. Cherryh
The Goblin Emperor by Katherine Addison
The Last Unicorn by Peter S. Beagle
Good Omens by Terry Pratchett and Neil Gaiman
The Hobbit by J R R Tolkien
Tarzan of the Apes (unabridged) by Edgar Rice Burroughs
Almost everything Terry Pratchett wrote is standalone. I heartily recommend any of his first 30 or so books - particularly Wyrd Sisters, Guards! Guards!, Moving Pictures, Small Gods and Pyramids. I warn you though, it will be very difficult to eat just one.
There are a number of small book trilogies (try Corum) by Michael Moorcock, an author many have forgotten.