I worked in cardiology, so this was a delightful read; exhilarating, in fact. Very technical, but extremely well written and informative. It's old, but always applicable, even though electrophysiology has advanced a great deal since it was written. I particularly appreciated the insistence that we work to understand the electrophysiological causes of ECG variations. Reading ECGs is actually the work of a detective-like mind - you have to understand that what you see on the paper is evidence of what is happening in the heart, and never forget that your interpretation could be incorrect.
At the time I read this book it was the definitive text on electrocardiology. The book is detailed and requires knowledge of physics and basic medicine.