So here's my own list. My If You Like... table, if you will. The books I recommend aren't exactly the same, but there are specific reasons for me choosing them; they have common threads that I think readers will enjoy. This post will feature contemporary series; future posts will feature stand-alones, authors, and a couple of big non-contemporary YA titles.
If You Like
The Ruby Oliver series, by E. Lockhart
These books are funny with a "quirky" narrator and plenty of hysterical drama. Many of the comparison books I've chosen also bring the funny, and often the same zany drama and personality.
serafina67: urgently requires life, by susie day - First of all this book is incredibly funny. But more than that it has, much like the Ruby Oliver books, true heart. Serafina, though not a "deranged mental patient," is quirky and confused and wonderful, much like Roo.
the espressologist, by kristina springer - Like the above series, this book has plenty of romantic drama and is lighthearted in the best way. It's a quick, feel-good read with a protagonist who gets into plenty of trouble of her own, much like Roo does.
the year of secret assignments, by jaclyn moriarty - This book focuses on close friendships and the trials good friends tackle together. But it's not so series. It's got its own brand of humor and characters who make Roo and her friends look positively normal. Plus, hijinks!
If You Like
The Sisterhood of the Traveling Pants series, by Ann Brashares
This series focuses on a tight-knit quartet of friends whose summers introduce them to love, loss, and a bunch of other really cliche-sounding, but really emotional and incredibly well-written elements of life.
peaches, by jodi lynn anderson - This series, which came out around the same time as the Traveling Pants has much of the same elements but trades magic pants for a decidedly southern setting. I'm honestly not sure which series I love more, but I do know that anyone who loved the Traveling Pants should be introduced to the girls of Peaches.
a little friendly advice, by siobhan vivan - This stand-alone novel has the same emotional depths as the Traveling Pants though it focuses more squarely on one character and her reliance, during a particularly difficult time in her life, on her three best friends.
the lonely hearts club, by elizabeth eulberg - One of the best "female friendship" stories, this novel sets up a positive, sometimes heartfelt, sometimes humorous story of one girl and her quest to build better friendships with fellow females instead of concentrating on romance. It's refreshing, sweet, and utterly uplifting.