If you like Forever in Blue:the fourth summer of the Sisterhood by Ann Brashares
Because you liked Forever in Blue: the Fourth Summer of the Sisterhood, you may also enjoy these books:
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.