There is the John Carter of Mars series in the library here at MB - Look for "Burroughs, Edgar Rice: Barsoom Omnibus, v3, 29th July 2009". Also some of Burrough's Pellucidar novels and Harry Harrison's Deathworld and some books by Charles de Lint, Clifford Simak, Murray Leinster and others. You look up those authors and see their books by series here -
http://www.fantasticfiction.co.uk/b/...ice-burroughs/ That's Burroughs for example.
Also the Baen free library has a number of free science fiction books -
http://www.baen.com/library/