I came across this book after having read John Milton's "Paradise Lost", which refers to a great war in Heaven, and Dante's "Divine Comedy", covering much the same thing ... while those two texts are centuries old, and written as poem, this book is new - and presented as a conventional fiction. It is well written and fascinating, and I recommend it to anyone interested in the topic.
The actual term - war in Heaven - is Biblical, found in the Book of Revelation. It's meaning is open to interpretation: Milton took it as the war that led to Lucifer being exiled to Hell just before God created the Earth. Most others view it as referring to the end of days, when Satan will build his forces and assault Heaven. This is the premise of the present book.
It's about a group of humans, most suffering in Hell, who are freed by the fallen angel Abaddon and mount an attack on Satan - who, all the while, seeks to defeat the forces of Heaven.
This book, while readable on its own, is the second in a trilogy. The entire trilogy is exciting, and well worth a read. While encompassing epic battles, it's also a story about love.