The Hollow Hills is the second book in Mary Stewart's Merlin Trilogy. Although it would undoubtedly stand alone itself, I would recommend starting with The Crystal Cave. This volume follows Merlin and Arthur from the future king's conception through to his coronation, and I thoroughly, thoroughly enjoyed reading it.
Stewart's style has Merlin telling the story as he remembers it, which allows her to gloss over or skip through time periods which are not wholly relevant to the plot. She does this with aplomb, giving us a brief spattering of anecdotes from Merlin's travels as Arthur, hidden away, grows up. Her second strength is in attempting to link the legends with historical facts - finding the real places to link to the stories which have entertained and entranced people for hundreds of years.
This doesn't mean she skimps on the magic, though, and Merlin as an enchanter, engineer, mathematician and general schemer is a believable and enjoyable hero.
This is not, however, a rip-roaring blockbuster of a book which will keep you glued to all the pages and constantly turning them. It is, however, a very comforting read which I look forward to when getting into bed every night. And that is something which I value very highly!