For science fiction, there's always Isaac Asimov's
Foundation series which is very good, and E.E. "Doc" Smith's
Lensman series, both from the old days, and more recently, Harry Turtledove's various series, starting with the
Videssos cycle on through the
WorldWar series and the
World at War series. And, of course, back to the earliest days of SF, Edgar Rice Burrough's
John Carter of Mars series. As a kid, I read science fiction by the bale lot, but almost every boy I knew read Burroughs and Asimov and Heinlein, long before we turned to Tolkien's
Lord of Rings and CS Lewis'
Narnia books.
I've also thoroughly enjoyed Patrick O'Brien's Aubrey-Maturin novels, and for non-fiction, Bruce Catton's Army of the Potomac trilogy and Douglas Southall Freeman's Lee's Lieutenants trilogy.