Besides the obvious classics, Tolkien, Herbert, Adams, etc., some sci-fi books I enjoy are these;
John Ringo's Legacy of the Aldenatta series, which includes A Hymn Before Battle, Gust Front, When the Devil Dances, and Hell's Faire. Also the Council Wars series with There Will Be Dragons, Emerald Sea, and Against the Tide.
E.E. "Doc" Smith's Lensman series (my name is from these) and Skylark series are both awesome, albiet dated.
Michael Crichton of Jurassic Park, Timeline (book was much better than the movie), and Sphere
Of course many many more.