After entering Area 7 you get the option to disenchant spells from Spellbooks in order to gain Essence. Essence is a resource that allows you to unlock specific spells.
When opening a Spellbook you will at some point enter the summary screen that shows all spells that you did get from the Spellbook you are opening. Here you can select the spells you want to disenchant by clicking on the red x in the bottom left corner of the spell.
Newly unlocked spells can not be disenchanted.