When pondering the question, "which book is the most important", I find that my answer after much consideration is a resounding: it depends. If the question of importance is clarified to query which book is most significant in making John Milton's "Paradise Lost" the important piece of literature that it has become, then any thorough reader will agree that Book I is easily the most important. However, if the question asked is which book is most important to ME as an individual reader, the answer without any doubt is the climax of the story which is found in Book IX.
Book I is the most important literarily because it provides the basic information and background one requires to not only understand the characters but also to understand Milton's intent in telling this biblical story based more in conversation and contemplation of its many players, than in physical actions.
The writer goes to great lengths to explain not only his character's actions and motives, but also to give insight into these motives by allowing readers an opportunity to consider the thoughts and past history which have shaped their personas.
Milton's epic poem begins in medias res (Latin for: in the middle of things) after Satan and his followers have been cast out of Heaven.
Satan's motive, which is based on anger and bitterness is simply to wage war against God forever as noted in lines 118 through 124:
Since through experience of this great event / In Arms not worse, in foresight
much advanc't, / We may with more successful hope resolve / To wage by force
or guile eternal War / Irreconcilable to our grand Foe, / Who now triumphs, and
in th' excess of joy / Sole reigning holds the Tyranny of Heav'n.
As Satan has awakened in...