calibre's regularnumbers are fine for me. I was looking at the actual book covers which use roman numbers , like IX for 9 and XI for 11
if you look at his amazon page, most covers have a legend of... number but some just say " a Drizzt Novel" so either he could not remember where it slots in or has left it as an exercise for the reader
E.G.
Timeless: A Drizzt Novel (Forgotten Realms: Drizzt) Kindle Edition
and finally, just when you think you have it all sussed he outputs books starring Drizzt which are not tagged with Drizzt at all but are a different series: e.g.
Hero (Homecoming Book 3) Kindle Edition
The saga of dark elf Drizzt Do'Urden .....