If you combine a skill book with another skill book then you get a brand new skill that requires both classes.
This only appears to work with an elemental and non-elemental skill book.
This is a quick list of the skills you can acquire this way.

Skill Book Combinations

• Fire + Necromancy = Corpse Explosion (explode a corpse, deal damage)
• Fire + Polymorph = Bleed Fire (enemies create fire surfaces when hit)
• Fire + Warfare = Sparking Swings (melee attacks leap to second target)
• Fire + Hunter = Explosive Trap (set up a trap, explodes when approached)
• Fire + Scoundrel = Sabotage (grenade or arrow in targets inventory explodes)
• Fire + Summoning = Fire Infusion (minion switches to fire and learns fireball)

• Air + Necromancy = Vacuum Touch (damage, suffocation and silence)
• Air + Polymorph = Vaporise (Turns surfaces into clouds and cures petrification and frozen)
• Air + Warfare = Breathing Bubble (Immunity to clouds and suffocate)
• Air + Hunter = Erratic Wisp (Target teleports randomly when hit)
• Air + Scoundrel = Smoke Cover (Create a cloud centered on self)
• Air + Summoning = Electric Infusion (minion switches to electric and learns eletric discharge)

• Water + Necromancy = Blood Rain (douses fires, makes enemies bleed, blood everywhere)
• Water + Polymorph = Healing Tears (generate tears that heal allies who move nearby)
• Water + Warfare = Cleanse Wounds (heal, remove burn, disabled, decay, poison and bleed)
• Water + Hunter = Cyrotherapy (convert nearby frozen ground into magic armour)
• Water + Scoundrel = Vampiric Hunger (50% life steal for two turns)
• Water + Summoning = Water Infusion (minion switches to water and gains restoration)

• Earth + Necromancy = Corrosive Spray (Cone of damage, acid and atrophy)
• Earth + Polymorph = Turn to Oil (Converts water/blood to oil and removes stuck and shock)
• Earth + Warfare = Oily Carapace (Consume nearby oil to gain physical armour)
• Earth + Hunter = Throw Dust (Damage, Blind, Removes surfaces and clouds)
• Earth + Scoundrel = Venom Coating (Adds poison damage to weapon attacks, 2 turns)
• Earth + Summoning = Poison Infusion (minion switches to poison and gains poison dart)

All skillbooks will be of the elemental element except for Blood Rain (which is necromancy) and all infusions (which are summoning).

All skillbooks require level 1 in each of the two skills needed to make them, except Corrosive Spray which requires level 2 in each.

Information provided on what the skills do is a summary of the tooltip, rather than actual ability use. In many cases the tooltip is a little vague.

Most amusing combination award goes to erratic wisp which you should use on your allies constantly.

More Guides:

• Divinity: Original Sin 2 – Unique Items in the Reaper’s Coast
• Divinity: Original Sin 2 – How to Play Singleplayer with Four Custom Characters
• Divinity: Original Sin 2 – Definitive Eternal Artefacts Guide (Where to Find Them)
• Divinity: Original Sin 2 – Difficulty Modes Guide
• Divinity: Original Sin 2 – Beginner Tips to Fight Guide (Mobility as Priority 1)

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