I think I have one more nomination available after seconding a couple of others. How about
:
A Fire Upon the Deep by Vernor Vinge
Subjects: Science Fiction, Science Fiction And Fantasy, General Fiction
Description: After a spaceship crashes on an unfamiliar world, a rescue ship races against time to rescue the downed ship's only survivors -- two children -- and retrieve the weapon required to prevent the destruction of the universe. This special eBook edition adds hundreds of annotations from Vinge which were written during the time of his original composition of this groundbreaking Hugo Award … more »winning novel. A Fire Upon the Deep won the prestigious Hugo Award when it was first published.
Not free, but available in eReader and Kindle:
http://inkmesh.com/ebooks/a-fire-upo...y+Vernor+Vinge