A high level Warrior Mage told me that at Circle 17 I should have 10 spells, though I only have 9, I've checked with Gauthus and he tells me I am not yet ready to learn a new spell. Is there an error or am I mistaken?

You'll get the tenth at 18th. Smack that mook that told ya different. ;-)

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That plan worked better in my head.