This one's easy: Corinna's life would still be normal if only her mother hadn't died—so the book is called If Only.
Lots of things would be different if only Sophie were still around: She'd still be best friends with Joci, she'd still be able to go to sleepovers, she'd still have someone to make her lunch, she'd still go to the dentist and pediatrician, and she might even have some new, cute clothes for her eighth grade year. She'd also have someone to ask about the changes in her body (or lack thereof) and explain what's up with boys. Most of all, though, she wouldn't be sad all the time… if only she still had a mom.