The book is largely biographical in nature, describing the life and work of the scientists involved in the creation of quantum theory. The rather reluctant quantisation of Planck, the work of Einstein, eager for a new look at the way the world works, and the struggles of Bohr to make it all consistent. Kumar looks at the work of Schrodinger and Heisenberg, and goes on to the arguments between Einstein and Bohr in the late 1920's and 1930's. The later chapters look at more recent work, such as that of Bohm and Bell.
My personal view is that the common idea that Bohr 'won' the debate is fundamentally wrong. Kumar doesn't go that far, but I felt he gave a more balanced view of the debate than many accounts, and that the book provides much useful material for anyone wanting to find the problems with Bohr's views. In any case, it's a well written book, giving a flavour of the development of an important part of modern physics without going into technicalities, and I would recommend it to anyone interested in the development of science over the last century.