A collection of sci-fi stories from the creator of Eat-Man, set in a world where people are still dealing with the repercussions of a war fought with aliens many years ago. We scanlated all but the first one-shot in the book (while it takes place in the same universe as the others in the book, the first one-shot is a stand-alone story and does not need to be read to understand anything about the others; see below for a summary of it).
A continuation of Blue Drop has appeared: Blue Drop ~Tenshi no Bokura~ (which also contains a separate story, Blue Drop ~Tenshi no Itazura~). It does contain some yuri, along with het and BL. See Baka-Updates for other people's scanlations of ~Tenshi no Bokura~ and ~Tenshi no Itazura~.
Blue Drop has also been made into an anime (~Tenshi-tachi no Gikyoku~), which takes place before ~Tenshi no Bokura~.

In "Blue Drop ~Broken Angel~," an ordinary high school girl finds herself in extraordinary circumstances. Will she fall in love with her protector?
"Kaijin" focuses on a pair of girls whose relationship is threatened by jealousy, bitterness, and forces beyond either of their control.
Yui turns up again in "Blue Drop ~The Angel's Promise~," where she fights to keep a former schoolmate from being forced to sacrifice her life.
"Blue Drop ~The Angel's Choice~" picks up where "Blue Drop ~The Angel's Promise~" left off, and reveals how exactly those two stories are connected to "Blue Drop ~Broken Angel~" and "Kaijin."

Here's a summary of the one story we didn't scanlate from this book, "Kaminoko" ("Divine Child"): On the island where the story takes place, a child with white hair was responsible for the deaths of a large number of people 100 years ago. The protagonist of the story is a high school girl. The boy she's in love with just had his hair turn white, and now everyone thinks he's going to bring destruction to the island. He used to be popular but is now shunned by his classmates. He comes to live with her, and they end up confessing their feelings for each other and sleeping together. Then he tells her the real story--about how he's being sacrificed to neutralize the residue from the thought-arresting weapons. Apparently so many people died last time because the child being sacrificed tried to run away. Therefore, the boy goes off and sacrifices himself, and the girl finds out she's pregnant with his baby.
A relevant quotation from the story: "A long time ago... there was a war with the aliens, right? Well, it turns out there's residue from the 'thought-arresting weapons' they used still buried in this island. They attack people, taking on the form of people's fears. Even when this residue is neutralized, it wells up out of the earth again 100 years later--it's a repeating cycle. They need human children to neutralize the residue, I don't know why. And I'm the one this time... apparently that's been a given since I was born. The reason so many people died 100 years ago was that the kid didn't want to do it and ran away--that delayed the neutralization."