I enjoyed the first book in the series immensely. However, I felt each successive book decreased in quality. I feel that if all three books (not including the novella) had been combined to make one book, they would have been stronger. I actually think some of the books themselves might be considered novellas and then the novella considered a short story. But, I'm no expert on those matters.
I read the novella out of curiosity and while the extra details were exciting, I didn't think the novella added enough to the over-arching story to be worth paying an additional price to read it.
Overall, I enjoyed the story and I think this series is worth a read but I would suggest against purchasing the novella unless you can get it in bundled pricing. Also, I would definitely try for an e-book or library copy as the paperback prices seem kinda steep. I see one goes up to $23 on Amazon.
You can click on the pictures to be taken to the Amazon page.