This article contains spoilers. Read at your own risk.
Dork Diaries: Tales from a Not-So-Happy Heartbreaker (also known as Holiday Heartbreak) is the 6th book in the Dork Diaries series. It was released on June 4th, 2013. 
Nikki is afraid to ask Brandon to the Sweetheart Dance. She is also afraid that before she gets a chance of asking him, he will agree on going with MacKenzie, the most popular girl of the school and Nikki's enemy. She gets a lot of mean texts from Brandon. Also, it was revealed in Tales From a Not-So-Smart Miss Know-It-All that she and her band, Dorkalicious, will record her original song, "Dorks Rule", with Trevor Chase. However, Trevor calls Nikki and tells her he won't be able to record with the band.
Oliver makes his first, and so far only appearance in this book.
Jessica does not physically appear in this book. However, she might of being either one of the CCPs Nikki mentions or in the background, making a background appearer.
In this book, it seems that Brianna has lost her irratational fear of the tooth fairy. Instead, she now has an obsession with diva hair salon shows on TV, calling herself "Miss Bri-Bri, fashionista-hairstylist to the stars", constantly bugging Nikki about it. This will continue through the books, and Nikki needs to stop it, like she did with the tooth fairy in Tales From a Not-So-Popular Party Girl.
Nikki reveals that Chloe Garcia and Marcy Simms begged her to take over her Miss-Know-it-all column for a month, with a special Crush Crisis Love Advice column, so she did. She almost sent a letter to them, asking why love is such a cruddy thing.
In this book, Marcy's name is accidentally spelled as Marcie.
This photo is a scene from the book where the main plot is unraveling.