If you like Second Summer of the Sisterhood by Ann Brashares
I'm guessing that you've already read the first one, The Sisterhood of the Traveling Pants (but if you haven't, I would really recommend going back to it).
Here are a few other titles that I think you may like:
Girls in Pants by Ann Brashares.
In the third book in the series, the girls from The Sisterhood of the Traveling Pants graduate from high school and spend their last summer before college learning about life and themselves. Also, if you'd like to continue with the series, the 4th one is coming out in January, it will be called Forever in Blue.
TTYL - by Lauren Myracle.
Chronicles, in "instant message" format, the day-to-day experiences, feelings, and plans of three friends, Zoe, Maddie, and Angela, as they begin tenth grade. This book also has a sequel, TTFN.
The True Meaning of Cleavage by Mariah Fredericks.
When Jess and Sari, best friends since seventh grade, begin their freshman year of high school and Sari becomes obsessed with a senior boy, Jess wonders if their friendship will survive.
What My Mother Doesn't Know by Sonya Sones.
Sophie describes her relationships with a series of boys as she searches for Mr. Right. This novel is in verse form, so each "chapter" is a poem.
The Boyfriend List by E. Lockhart.
A Seattle fifteen-year-old explains some of the reasons for her recent panic attacks, including breaking up with her boyfriend, losing all her girlfriends, tensions between her performance-artist mother and her father, and more. This book also has a sequel called The Boy Book.