A World at Arms: a Global History of World War II
Second time I read this book. They don't get any more comprehensive than this.
It is not a military history as much as a diplomatic, and global analysis of the conflict. It tries to explain in detail how things got to happen, and why, rather than what exactly happened. Geopolitics is the best word that defines this book. It assumes that the reader knows what happened, 'grosso modo', and then aims at the big picture, a great canvass that includes all countries and regions involved.
And it achieves this aim wonderfully. Now, you would like to dig more deeper -for sure- in some campaigns or countries more than in others: then you have to go to other sources.
Around 900 pages, and about 200 pages of notes. The only insufficiency that I find is the lack of illustrations, and photos. There's nothing but text here. Well... and a few poor & sketchy maps at the very back of the book.