peckrob

Foundation by Isaac Asimov.

Everyone I've given it to has loved it.

The whole Foundation series is wonderful

I really had to slog through Green Mars and Blue Mars. Red Mars was fun and interesting

Another hard sci-fi staple.

the Xeelee Sequence books spring to mind.

Manifold trilogy (Manifold: Space, Manifold: Time and Manifold: Origin) are also fantastic.

The rest of the Rama books he "co-wrote" with Gentry Lee are decent but become more space opera than hard sci-fi.

Vacuum Diagrams by Stephen Baxter.

I love stories like this one: primitive, longshot interstellar exploration and primitive, first generation colonization.

The first two Coyote books were good, but I just can't get into any of the subsequent ones.

Flight of the Old Dog is another favorite of mine by him