Just a few years ago dark matter and energy seemed highly speculative ideas. Hence the announcement that the percentages of normal matter, dark matter and dark energy had been found fairly precisely came as something of a surprise to me. In Alpha and Omega Charles Seife
looks at the discoveries which brought this about, which he calls the third cosmological revolution. Despite the title it's really about &Lambda - the cosmological constant - and &Omega - the density of the universe, and what these mean in our view of cosmology. The book is highly readable and is aimed at a non-technical audience, but there's plenty of information on the latest (in 2003) experiments, so more knowledgeable readers are still likely to find it of interest.
The book starts with look at the history of cosmology, dealing with the first (Copernicus and Galileo) and second (Einstein) cosmological revolutions. I did think that 'in between' revolutions the thread got a bit lost, in particular describing the work in the middle of the 20th century. The bulk of the book looks at the various candidates for what makes up the dark matter and energy, including Neutrinos, WIMPS and MACHOs There are also chapters on supersymmetry and the search for gravitational waves, as well as several appendices, including one with a list of ongoing experiments related to cosmology.