The light novel is the official prequel to the anime series. 
It tells the story of how Yuri and Hinata meet, how they find out about "Angel" and the other characters and how the SSS was founded.

In a world after death, angels fight for their fate and their future. Yuri, the leader of the Shinda Sekai Sensen, rebels against the god who destined her to have an unreasonable life. On the other hand, Tenshi, the chairperson of the student council for the world after death, battles against the SSS members. SSS members utilize armed weaponry to battle it out against the angels harnessing supernatural powers. 
Angel Beats! Heaven's Door is based on the the novel Angel Beats! Track Zero.

Otonashi wakes up in the afterlife to find out he has no memories of his life before death. Desperate to survive in a war that could yield another unfortunate fate, he joins the SSS in the battle against Angel, whose very mission is to destroy all inhabitants of this afterlife. As Otonashi struggles to recover his memory and learn more about this world, he finds that not all is as it had seemed. 
(Source: Funimation)

Kanade Tachibana is a beautiful girl known as "Angel-chan" at school.
She is traveling all over the country in search of "someone."
One day, while staying in Kanagawa, she met Yuri Nakamura, a happy and cheerful girl.
 
Along the way, other people who look like SSS members (?) also appear⁉
Will Angel-chan be able to find that special "someone"…?
 
(Source: ComicWalker, translated)

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