The next three volumes include fish groups that I would think would spur good demand, at least relative to volume five. Maybe they don't think that they can set a price point high enough to make them profitable. An incomplete set really irritates a compulsive person like myself.
The majority of people in and out of this hobby would rather spend $50 on a fish that will probably die, and another $509 replacing it then spending $50 on a book full of great useful information.
Sadly, that is very true. To make it even worse, many stores support this behavior.
I refer to this series constantly, and own all the other volumes. As much as I enjoy having a book in hand, I thought that the publisher would at least offer the last volumes as e-book editions. I have been coveting the volume on gobies since the series began.
Let the publishers know you want these books!