As far as I know it is a set selection of spells (not in any kind of research grouping).
As evidence I point to the fact (A) that all indie spells can be seen at any point in the game and (B) that in a recent patch note 'Indep grimoire slightly improved.' (that is from July 27th on the dominions progress page,
here.
I am not sure if this is denoted by name or ID (in case you wanted to mod the spells), but I would guess ID (just on a hunch and a little hope).