Title: Shoujo no Kisetsu
Mangaka: Saki Kaori
ISBN: 4773006226

I think most yuri fans know of Saki Kaori because of her manga Haru yo, Koi, which is being scanlated by Chaotixubs.

The yuri one-shot from this volume is "Ano Natsu no Joshou" ("That Summer's Prologue"). I like the story (though not its message about bisexuals), and the sex scene isn't too long.
Some Yuri Group has scanlated this one-shot (though they've mistakenly labeled the one-shot itself as Shoujo no Kisetsu instead of "Ano Natsu no Joshou"). Torrent can be found here.

The main character, Emi, tells us that she fell in love in the summer of her first year of high school (10th grade in the US), and that it was a love that would dominate her entire life.

Emi's friend, Ryouko, invites her to spend the night. Emi and Ryouko met each other that spring and became friends, and now they're always together.

Ryouko tells Emi that because her parents are divorced and her father's always working, it's almost like she's living alone. Emi cooks them dinner (Ryouko doesn't cook). They both drink beer and end up getting drunk.

Ryouko asks Emi if there's a guy she likes. Emi says there isn't, because she goes to a girls' school and doesn't have any opportunities to meet guys. Ryouko says it's a waste, because Emi would definitely be popular with guys--she's cute, has good style, nice hair, etc. Ryouko says that if she were a guy, she wouldn't leave Emi alone. Then she kisses her.

Ryouko tells Emi she's been thinking for awhile now that Emi is cute. She asks Emi if she's angry, and Emi says no. The seduction continues.

Emi tells us that she slept with Ryouko that night, and that she, Emi, was the one who was really crazy about the relationship with Ryouko.

After the sex, Ryouko tells Emi that she's talented--she's gotten really good at it. She tells her that she (Emi) is better than the guy she (Ryouko) went out with before.

Emi is surprised that Ryouko went out with a guy.

Emi tells us (again) that she was crazy about Ryouko.

Emi asks Ryouko if she can come over that day, and Ryouko says no--she has a friend coming over. Emi asks who it is, and Ryouko says it's none of her business. Emi protests, and Ryouko asks if she's jealous.

Emi tells us that, unable to think of anything but Ryouko but knowing the reverse wasn't true, she fluctuated between joy and sadness.

A man answers the door at Ryouko's place and asks Emi who she is. Ryouko comes up behind him and asks Emi why she's there.

Emi asks why Ryouko is going out with a guy, and tells her that she's serious about her. Ryouko asks why Emi is so serious--all they did was sleep together, after all. She tells her that she likes guys too, so there's nothing she can do about it.

Emi kisses Ryouko, but she leaves, saying she's fed up with Emi.

Emi tells us that even though Ryouko was just fooling around, their relationship was very important to her (Emi), though she hadn't realized it herself at the time; it was because she realized she could only love women.

We skip ahead to the present, where Emi is hitting on her kohai (a person who hasn't worked at the company as long as she has), telling her that she's been thinking for awhile now that she's cute (the same line Ryouko used on her!).

She tells us that the love she experienced that summer was a prologue to her true desire.