It isn't the last in the series - it is number 6. Numbers 7 8 and 9 are not yet available in Kindle editions but I'm sure they will be eventually. 10, 11, 12, 13 and 14 are available as Kindle editions and 15 - the latest one currently will be available later this year.
The only ones not yet available are:
7 - Murder by Bicycle
8 - Murder of Identity
9 - Murder in the Park