If you're looking for manga similar to Angel Beats! Heaven's Door, you might like these titles. All recommendations are made by Anime-Planet users like you!
When you think of the plot series of Angel Beats some how it just translates right away into The Melancholy of Haruhi Suzumiya. The whole entire feeling to the series is similar with a group of kids in a school setting fights against supernatural phenomenon. The lead heroines are spitting images of each other putting on a gutsy attitude while leading the protagonist around in a world completely different from what they are used to.
Both Angel Beats and the Melancholy of Haruhi Suzumiya just seem to go well together. They both have unique groups of kids that interact with mysterious entities. Each has a male protagonist that is pulled into the heroine's world against their will. The heroine's themselves are very similar to each other, both in personality and physical stature. They both have very funny moments and are quite enjoyable.
This is a pure gut recommendation since I will never real the Angel Beats manga. But Futagami Double gave me a similar feel because of the anime. Supernatural students fighting against something that is of a higher power than them but they might possibly defeat it. Just read Futagami Double if you liked Angel Beats and vise versa. I'm sure you'll enjoy.
There is an emotional connection to the characters in both Clannad and Angel Beats. Trying to fight the imperfections that exist in each of their worlds and strive to overcome the obstacles standing in their paths. Finding love as well as friendship and experiencing loads of dramatic moments that will leave you wanting more at the end.