I have thoroughly enjoyed nearly every book written by Andre Norton, but some of my favorite sci-fi titles of hers are Star Man’s Son, Star Rangers, Galactic Derelict, Voodoo Planet, The Stars are Ours!, The Zero Stone, and Lord of Thunder. If you have a used-book store in your area, odds are very high that they have at least a handful of Andre Norton books on the shelves; thumb through a few and see what you think. 🙂
I know these have already been mentioned, but I’d also give a big thumbs-up to David Brin’s Uplift Wars series, David Weber’s Honor Harrington series (the first book is On Basilisk Station), Anne McCaffrey’s Pern and Crystal Singer series, and C.J. Cherryh’s Foreigner series. Very, very good authors all!