When you make a sovereign in 1.09e you have the choice between the four elements and the enchantment spellbook.

However sovereigns made in earlier versions can have the life, death and summoning spellbooks.

Those sovereigns can still be picked in a game, and it looks that the old books are empty (at least death).

Still, I do remember some spells (summon bear) that seemed specific to the summoning spellbook.

To be honest, all books need more love and more spells. And also I would like to know from the spell from which book it comes.