Jack Glass and The Thing Itself, but every one of his books I've read has been excellent, so it seemed time to start filling in the gaps.
I went for By Light Alone because of its interesting sounding premise. It's a cracker (as they say). The idea is that science has produced a mechanism where people can get all the energy they need from sunlight, thanks to a bug that turns their hair into super-photosynethic light absorbers. All they need to live is some water and a few essential nutrients. A clever (if technically verging on the impossible) idea, certainly. But where Roberts triumphs is in going into the unexpected implications of the change - the absolute heart of what makes science fiction, and which so few literary types who do SF down, and think it's all about spaceships and ray guns, appreciate.
One implication considered is that for the first time ever it's possible to have a group of people who have literary no money at all. Not just poor but literally penniless. Roberts also examines the possibilities for male/female distinctions, and how a small group of wealthy people might consider those who have the special hair to be a subspecies, and to conspicuously wear their hair short to emphasise they don't need it.
The book is divided into four parts, each seen from a different (but linked) individual's point of view. At the heart of the book is the story of a privileged family whose daughter is taken from them on a skiing holiday. They assume initially it is as a hostage, but the authorities gradually explain that something much darker is behind it.
The one fault I would say that the book has is that the forth segment, which is the longest, seen from the viewpoint of the captured daughter, is the least effective. It's partly because the environment she is in forces a slow, plodding development, with occasional dramatic outbreaks of violence, but also because it just doesn't work quite as well as the other sections. It's good, but the others are brilliant.
If you want to see what good, modern science fiction is like - or are looking for a new author to branch out into, Adam Roberts is an obvious choice. I wouldn't go straight to his latest, The Thing Itself, as it is his most complex book, but either By Light Alone or Jack Glass would make an excellent way in. Recommended.
By Light Alone is available from amazon.co.uk and amazon.com.