If you take a look at the objects file (Divinity - Original Sin\Data\Editor\Templates\Stats\object.xslm), you can see what skillbooks can be found at what level.
First check the ActPart, this is the "level of the skillbook", by default you will only find the item if your character is min ActPart-1.
Secondly, check the MinLevel and MaxLevel fields at the right hand side, this is an overwrite for when the item can be found, ignoring the actual item level.
As you can see: The ressurect skillbook is a level 16 item, but the min/max overwite states that the item can be found when you are between level 14 and level 20.
The whirlwind book for example can only be found when you are minimum level 6, since the item is level 7.
Next up we check the ObjectCategory to see when/where it can spawn.
As you can see, SKILLBOOK_Warrior_Target_StunStart has a ObjectCategory SkillbookWarrior.
If we head over to the treasure table file (Treasuretable.xls), we see that SkillbookWarrior can be dropped by warrior trainers.
However, the ObjectCategory of the whirlwind skillbook item is blank which means that it will not drop or be sold by random generation. This usually means that this item is placed somewhere in the world for you to find deliberately, or you receive it form someone upon completing a quest.
