It's impossible to mention Mary Anne and the Great Romance without also mentioning Dawn's Wicked Stepsister. The two books work as a pair, and Mary Anne's story in particular does a terrific job of exploring the mix of excitement and apprehension that comes with joining a blended family.
In the first book, Mary Anne's dad and Dawn's mom decide to get married, which means the best friends will now be stepsisters too. But it also means Mary Anne has to deal with the reality of moving, sharing her dad with Dawn and her new step-mom, and generally having her entire life change in a matter of days. For any child struggling with new blended family dynamics this book and Dawn's Wicked Stepsister, which continues to chronicle Dawn and Mary Anne adjusting to their new living arrangement, are must-reads.