This book is an excellent account of the accident. Besides being personally knowlegable, the author quotes NUMEROUS eye-witnesses to the disaster. He goes minute-by-minute (sometimes second-by-second) with virtually all the individuals who were present during the accident, and the politicians who, before and during the accident, made things worse.
However, in order to fully appreciate this book, some prior knowlege is needed. For example, terms like roentgens (a measure of radiation exposure) were never explained in laymen's terms - although even a layman can understand that, as the author points out, an instrument whose scale only goes up to 3.6 roentgens is inadequate to measure radiation in the range of 20,000 roentgens! Thus, most of the most important facts are fairly easy to deduce from context, although a glossery of nuclear terms would have been helpful.
Because the author has such a detailed knowlege of the subject, his account can occasionally loose the forest for the trees. For this reason, I say that it is an EXCELLENT second book to read about the disaster. If you already know the outlines of the events and have had the major terms defined for you (the "forest") by some other book, you cannot find a better book to explore every "tree" in detail. You don't need a physics doctorate, just some basic background.
But, even without any prior knowlege - my situation - the author's writing style is excellent. He captures the drama and the heroism with crackling intensity. He jumps from person to person, all around the plant, but he keeps the context, so the reader can see all these diffenerent groups and individuals working desperately in lethal conditions. And his pacing is excellent. Every person's experiences are described in detail, yet no one's account is sacrificed for anyone else's.
In conclusion, go get some basic background first, then READ THIS BOOK.