Green Lantern #64
(Spoilers abroad in here, so stop reading Ken)
War of the Lanterns begins with this issue and spreads across all three Lantern books over the next few months. As this issue begins, all we really have known is that the renegade Guardian Krona is behind it but by end we get a much fuller storyline. As Hal Jordan and the New Guardians (Sinestro, Saint Walker, Carol Ferris, Atrocitus, Larfleez and Indigo-1) look for Krona and the Entities he has stolen, they come across the Book of the Black, finding it's more than they ever thought. As that happens, Krona has taken the Entities to the Guardians of Oa. Showing them what true emotion is, he has six of the Entities take them as their hosts, leaving Parallax to enter the Central Battery, taking control of all Green Lanterns. We now know why the Lanterns are at War with each other.
Not much can be said for this book that I haven't already said. Geoff Johns is amazing. This book is amazing. You absolutely should be reading this is you have even the slightest interest in comics. I do not see how you can be disappointed with it.