After I read Ready Player One by Ernest Cline, I told myself I would start reading more science fiction books so my first pick was this book. And I'm very much glad I did. I found it much more fascinating than RPO.
The basic premise of the novel is that humans are fighting to colonize space and recruits enlist on their 75th birthday. Because of laws that were passed, no one on Earth actually knows what it's like in space, from what aliens are like or how the Colonial Defense Forces turn 75 year-olds into top fighting form (or even WHY they want people at that age). The main character is an old white male who signs up on his 75th birthday. That's all I'll say without spoiling anything.
I read it one day, staying up until 5am to finish it. It was fascinating (though not really a surprise when Neil Gaiman recommends Scalzi and others say he's a master of the genre).
I gave it 4 instead of 5 because I thought the ending was very abrupt.
My library has the rest of the series so I'm probably going to check those out and read them.