One of my favorite middle-grade books is It's Raining Cupcakes by Lisa Schroeder (who also happens to be one of my favorite authors.) It's fun, cute, and the perfect read for any time of the year. Tweens, particularly girls, are sure to fall in love with the story and lucky for them, a companion will be coming out next year.
Models Don't Eat Chocolate Cookies by Erin Dionne. I reviewed this book back on my old blog and it's still a favorite of mine. It's great for any tween and even better for those tweens that are starting to become weight-conscious.
The Wedding Planner's Daughter (and sequels) by Coleen Murtagh Paratore. I fell in love with this series a few years ago and even though I'm technically to old to enjoy it now, I still find myself waiting for the sequels. These books are laugh-out-loud funny and a quick read. Paratore's other book, The Funeral Director's Son, is a great pick for boys as well.