Heinlein's books are very readable and not too long, Tunnel in the Sky, or Have Spacesuit Will Travel are fun if you enjoy a retro sci-fi feel.

Rendezvous with Rama is a great story of exploration, and if you like that, check out Ringworld, and The Mote in God's Eye.

A Fire Upon the Deep, Hyperion, and the Culture novels all have some really cool concepts and a lot of space travel

Old Man's War is a fun read if you like a military story