Ruby Red by Kerstin Gier

1st in Ruby Red series.  Sequel is Sapphire Blue.  First published in Germany.

Gwen is 16 and lives with her mum, siblings, grandmother, aunt and cousin in an upperclass London neighborhood.  Her extended family doesn't pay much attention to her, focused on her cousin, believed to be the carrier of the time-travel gene that runs through her family and sure to show up very soon in the form of an abrupt trip to somewhere in the past.  But all is not as it seems.  Since a mathematical formula had worked out that Gwen's real birth  date was the birth date of the next time traveler, and her cousin was also born that day, Gwen's mum and (late) father secretly falsified her records to indicate Gwen was born the day before, allegedly in the hopes that she would be able to live a normal life (and since she was born at home with a midwife, no one was the wiser.

The False Princess by Eilis O'Neal

Nalia spends the first 16 years of her life thinking she is the princess and heir to the throne of Thorvald, only to be told that she was only a decoy, switched at birth with the real princess to try to avoid a prophecy made during the queen's pregnancy that the princess would be murdered horribly before her 16th birthday.  The king and queen, having averted their real daughter's death, send the false princess, named Sinda at birth, unceremoniously away from the castle to live in a small village with her only living relative, an aunt who is less than delighted to have a fully grown niece with no domestic skills sprung on her with no means of support.

Only the Good Spy Young, by Ally Carter, Bk 4 in the Gallagher Girls series

In the previous three books in the series Cammie aka Chameleon Morgan has found out how serious being a Gallagher Girl is.  In the beginning it was fun knowing that she was a spy-in-training, sneaking out to meet a townie boy in Book 1 and using their special skills at detection, evasion, invention and deception to learn more about the mysterious boys from Blackthorne Academy in Book 2. 

It's a Mall World After All by Janette Rallison

Charlotte has a grudge against her best friend's boyfriend because of how he treated her in middle school, teasing her and making her life miserable.  As seniors now, Charlotte is convinced that Bryant is cheating on Brianna, and spies on him to prove it.  Instead of finding hard evidence of cheating, she keeps getting herself in embarrassing (and messy) situations, usually involving Colton, a guy in many of her classes, president of the National Honor Society club she belongs to, and also the guy who is Bryant's best friend and convinced he isn't cheating.  Charlotte has to face her own prejudices against Bryant and hope Colton and Brianna can get past their preconceived notions of Bryant's honesty in a light and fluffy book about high school and friendships.  A sub-plot in which Charlotte helps a little boy get shoes for his mom by organizing a Christmas gift drive offers some real heart, and Rallison's writing offers real opportunities for laughing out loud.

SLJ recommendation for grades 6-10

The Disreputable History of Frankie Landau Banks by E. Lockhart

Frankie has her growth spurt and discovers leave-in conditioner the summer after her freshman year in high school, and when she gets back to school, boys seem to notice.  In particular one boy, Matthew, whom Frankie begins to date.  But Matthew and his friends are part of a secret society, the Basset Hounds, and the society doesn't let in girls.  Frankie, frustrated at being underestimated, impersonates the Basset's leader, organizing increasingly daring and successful pranks on campus, but pulling off brilliant pranks while letting someone else gets the credit rankles, and ultimately she has to come clean.  There are elements of romance, but ultimately this story is about Frankie and her growing realization of how being female effects how others see and treat her.  It doesn't have a "happy ending" in that Frankie does not end up with the boy of her dreams, but she does recognize her own self-confidence, worth and sense of power.

One Night That Changes Everything by Lauren Barnholdt

Eliza is looking forward to a quiet night watching movies with her best friends while her parents are out of town, but a club of boys from her school, including the ex-boyfriend that only dated her as initiation into the club, use her notebook of secret things to blackmail her into doing stunts around Boston. Includes under-age drinking, references to drugs and sex that might make some teens (and unparents) uncomfortable, but the overall message doesn't glorify any of them. The relationships with best friends are depicted pretty realistically (both friends get distracted by their own concerns - other friends, boys, etc. - but pull together for Eliza when she needs them most. A touch of romance, a supportive relationship with her older sister, and the thrill and danger of staying out all night will appeal to teen readers, probably of the female variety.

My recommendation - 9th grade and up.