I am yet to encounter a single SCW (Spanish Civil War) historian who doesn't put this on their "essential" side of a bibliography. It allows English-speaking students and those interested in the war an irreplaceable gateway into individual stories of the war and revolution. At the time - and to this day - it is landmark in its scope and content.
I can sympathise with the other reviewer a little. The structure is confused and complicated. It is not a book to read cover to cover really; it is one to follow in conjunction with other histories and to pick up where appropriate. For instance, it has encounters and interviews punctuated by Fraser's own take on the war in a piecemeal and sometimes broken format. Frequently, protagonists appear at different times in the book (capitalised helpfully) only in passing and you can easily lose focus. The segmentation of the book is often frustrating.
That said the book is truly magnificent in supplementing general and more local histories of the war. The core of the book is segments of interviews sewn together with Fraser's narrative across a whole scope of stories of participants: from the highest commanders to the lowliest braceros (landless labourers), its breadth is its main feature. You can pick the book up and read about personal experiences from a variety of political viewpoints, areas of Spain and during the whole process of the war, from the demise of the Republic (February 1936) to the capture of Madrid (April 1939) by Franco. The sheer variety of interviews and the "off the beaten track" way in which Fraser constructs a grand narrative of many smaller ones is without match in the historiography of the conflict in English.
This is not a book for all. Those with a passing interest may become confused and frustrated by the implicit knowledge that is often required, and the sheer weight of information. Students and obsessives will find the book without match in English. It allows access to a good amount of primary testimony, but contextualised and from across the political spectrum. After around five years of studying the war I still regularly refer to, and use the book. Not without its faults, but an absolute must for those without sufficient Spanish skills.