This is the fourth book in a series--the numbering starts at 0 (the first book is a short free introduction). I've thoroughly enjoyed the entire series and will read subsequent books if they are written. While this book brings this series to a natural stopping point, it doesn't preclude additional serials.
Readers should read the books in sequence, as there are several plot assumptions and references that just won't make sense if you read the books out of order. There are some unbelievable plot twists in Book 2 that might deter further reading, but don't stop, as the series quickly gets back on course with Book 3 and is a joy to read.