'A battle in Heaven' is about spiritual warfare in which the Church of Jesus Christ is involved against the powers of darkness. The Bible says:
For our wrestling is not against flesh and blood, but against the principalities, against the powers, against the world-rulers of this darkness, against the spiritual [hosts] of wickedness in the heavenly [places]. (Eph. 6:12).
It is not a fight against a people, or a person, but against the fallen angels and evil spirits in heaven (fallen angels are not in hell, Rev. 12:7). The Church will surely win this battle, but not before she will be obedient to her Lord and takes her role.
Unfortunately, most of the people of the Church of Jesus Christ are not aware of their position and are losing battle after battle against the great deceiver and liar, the Satan. The Church doesn't know the enemy and doesn't want to engage in this battle, under the slogan that she doesn't want to focus on the enemy. This disobedience to her Lord, by not engaging in the spiritual battle, has disastrous consequences: many shall depart from the faith.
This book is not a theory about the meaning of spiritual warfare, but a report of that battle in a certain area.