This is not actually part of the series ARIA, but I couldn't think of a better place to put this, so I'm listing it under ARIA.
It contains 5 short stories, "zenyasai," "ichigo-chan Panic," "setsuna no natsu," "chiisa na seiya (Petite Noël)," and "mukuukai." They're all fairly straightforward feel-good type stories. You can really tell where the themes for ARIA come from. "Petite Noël" is even about cats. Amano-sensei must be an optimist, and a somewhat simplistic one, too. I'm not necessarily saying that it's a bad thing.
On an unrelated note, the "ichigo-chan Panic" in this book has nothing to do with the currently airing "Strawberry Panic" radio and the soon to be starting YURI anime of the same title.