Almost anything (scifi) by Andre Norton, the best being Sargasso of Space, closely followed by Moon of three rings, and Forerunner
Citizen of the Galaxy by Heinlein, A boat of a million years by Poul Anderson
Larry Nivens Destiny's Road, Jack McDavitt's Eternity Road, or the Engines of God(Awesome book), Glory Season by David Brin, Foundation, Foundations Edge, Second Foundation, Prelude to Foundation, Forward the Foundation Isaac Asimov, The Robot Trilogy by Asimov, Any of the old Ace double books the best being Robert Silversburg The Suncatcher, of course Battlefield Earth if you want a long read, Hard SF SpO Red Mars, Blue Mars, Green Mars by K. Robinson (?), the Dispossed by Ursula K. Leguin.
Andre Norton was one of the best of the quiet authors, she is the first person whose books I read literaly any of her books are worth reading, The Zero Stone, Postmarked the Stars, Plague Ship, Jargoon Pard, Operation Time Slip, Forerunner, Forerunner Foray, Moon of three Rings, The Stars are Ours, Star-born, Starman's Son (Daybreak 2500AD), Star Patrol, Brotherhood of the Shadows all of those are great space opera's in the purest sense of that form
Just thought of another great space opera author Murray Liensters Med Ship Series is awesome