In the Beginning by Peter David

From the publisher (source: The Internet Book List):
The Battle of the Line is infamous in Earth history as the climactic campaign in a long and bloody war between Humankind and the alien Minbari race. But how did this apocalyptic conflict erupt? And how did this fateful clash of two civilizations ultimately lead to the creation of the Babylon 5 space station? Here, at last, the great space epic unfolds - in a new novel based on the first full-length Babylon 5 television movie.

Eager to expand their trade with other worlds, the people of planet Earth attempt to make contact with the mysterious Minbari, a race of proud and powerful warriors. But when communication turns to violence, Earth becomes the target of a latter-day holy war, waged by a civilization possessing vastly superior numbers and firepower.

And the Minbari will stop at nothing short of the total extermination of its enemy: the Human race...

This book is part of the "Babylon 5: Episode Novelizations and Scriptbooks" series.
Here are some other books from this series: