The major topic of The Philadelphia Chromosome, the discovery of a cancer causing genetic mutation known as chronic myeloid leukemia (CML), was actually touched on briefly in The Emperor of All Maladies. However, Jessica Wapner uses the entirety of her book to focus on the connections between genetics and cancer, from the late 1950's through a series of breakthroughs that have allowed thousands to survive what would have once been a fatal diagnosis.
The Philadelphia Chromosome works well because of its balance. It never reads like an overly technical textbook, nor does it fall into a wordy history or weepy biography. Wapner uses the personal stories of patients who were able to survive by taking a daily pill to piece together the history of genetic cancer research. While the science behind the success is described, it is broken down in a way that readers unfamiliar with the details of genetics can easily understand.
Though parts of the book did read rather similar to The Emperor of All Maladies, I didn't necessarily feel like I was reading the same information over again. In fact, I would recommend reading it prior to reading The Philadelphia Chromosome, since it provides readers with a good groundwork in the history of cancer research that Wapner's book can easily build on.