I'm a fan of this serie and is literally the only serie I've read.
This book has a great intro and is a great book.
If you look at this book and compare it to the most recent one (13), you can say it's improved. Not only the writing, but also the illustrations.
In this serie (and in any other serie) its better to read them in order. I haven't read them in order and I should've. There are a few references from the book before. For example: If you read the 13 before the 12, you don't know that Nikki had an all-paid trip to Paris (mentioned in book 12). So I recommend you read them in order.
I wish for the series to go on.