I first read about Orson Scott Card's Ender's Game, when commentary about the movie started appearing on the web. Intrigued, I read the first chapter at Exclusives, and then got hooked - so got the full series.
While it is sometimes a straight forward science-fiction novel, what I really like about the series, is the intersection of philosophical discussions on alien interactions - something that is not only applicable with regards to interactions with other sentient species (whose numbers are now far more probable) but also species that seem to be non-sentient. It effectively turns the idea of animal rights on its head - does the fact that humans don't understand communication of other animals on Earth imply that they are not sentient - but rather that we merely can't communicate with them.
The other key idea is the concept of life itself - specifically, when should a computer program, without a physical presence be considered alive. I did not particularly like the eventual descent into metaphysics of what is the soul - but the very idea of an artificial program that could be alive and is alive in all senses but for the fact that there is no physical body; is a great construct.
It is a brilliant series, and the series had amazing foresight - given that many of the technologies such as the Internet, mobile devices, social media etc. did not exist (or existed only in a very early form) when the series was written. I especially enjoyed the second (Speaker for the Dead) and third (Xenocide) books - and the full series is well worth reading.