The Forever War

The Forever War - Joe Haldeman

Young William Mandella has been conscripted by the UN in humanity's war against the Taurans.

However, due to the nature of interstellar travel and Einstenian rellativistics, he only ages months while on Earth centuries pass.

The book is divided into sections called "Private Mandella", "Sergeant Mandella" and so on, chronicling his climb up the ranks -- more from luck than skill or leadership. Eventually he and his true love, Marygay Potter become the "oldest" active combatants.

Mandella has trouble fitting into society, because of the changes in language, ethnicity and sexuality over hundreds of years, which to him, feel like only four.
Eventually he is reunited with Marygay and they begin a life together.

This book is sometimes considered an antiwar response to Robert Heinlein's Starship Troopers; it has also been said the book is the author's narrative of his Vietnam war experience with a Sci-fi veneer.

I enjoyed it very much, and I am on the lookout for A Separate War, by the same author, which tells Marygay's story of the time she was separated from Mandella.

UPDATE Oct 20th, 2008:
Ridley Scott has acquired the film-rights to Joe Haldeman's magnificent, Hugo-award-winning classic science fiction novel, The Forever War.
Via Boing Boing

ISBN 0-312-29890-0

Timescape

Timescape - Gregory Benford

Reread this one recently.
Published in 1980, it is a time travel story (of sorts), where a team of scientists in 1998 attempt to send messages to 1962 so mankind avoids certain global actions that cause an enviromental collapse in their time.
The catch is they have to avoid the grandfather paradox; they want to avoid the enviromental collapse, but without changing history so much that they eventually don't send the message back in time.
Considered "hard" sci-fi, the story sometimes plods along, and there is no climax to the story in itself, until the very end.

3 out of 5