I was skeptical of the middle grade play with Notebooks of a Middle School Princess, and trying to age up an existing series is strange in and of itself. This book pretty much just takes place five years after the entirety of the initial YA series and covers exactly what the title entails. Like all the Princess Diaries books, it's fairly straightforward and dives right back in.
The book is basically a warm blanket with a bit of an adult twist. It's decidedly PG-13 and is ultimately less scandalous than the YA the series introduced so many readers to, but the really interesting part for me was how it wove itself into the middle school timeline so well. The middle grade book actually happens in the same timeframe as this book does, and it means that, for an adult reader, the middle grade book almost feels like a tiny bonus feature. It was a neat little thing Cabot did that made me appreciate both books more as a result.
At the end of the day, you'll know if you're going to like this before you even pick it up. If you liked the original books at all, this doesn't feel like a needless add-on, and the fact that it's actually pretty great doesn't hurt, either.