I've just finished reading Old Man's War by John Scalzi - a science fiction novel in a similar vein to Joe Haldeman's Forever War. Old Man's War follows the life (or afterlife if you like) of John Perry, a 75 year old widower with testicular cancer (though he doesn't know that bit yet) who signs up to the CDF - Colonial Defence Forces - in a bid to prolong his life besides the fact that he's really got nothing better to do any more.
The CDF represent the armed forces of humanity as they battle against myriad alien races to colonise space and protect the human species. Cloning, nanotech and just about every (presently) ethically questionable technologies are commonplace which explains why Perry, along with a host of other geriatric men and women voluntarily drafted themselves in. Their minds are transferred into their cloned and heavily upgraded bodies after which they are rigourously trained before being assigned to a military unit fighting battles on a variety of fronts against the aforementioned alien races.
John Perry's story involves action, strategy, technology, humour, sex, morals, ethics and a lot of involuntary page turning on the reader's part. I highly recommend this to any sci-fi fans out there.
I've already started reading its sequel - The Ghost Brigades - and suspect that I'll be reading the next book in the series too.