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Depends who you ask. For someone skilled in English, that pluralisation is fine. Based, on my experience as a teacher, kids would find the es pluralisation is simpler, though I'm not saying that...

Look, if we want to be prentious arses about this, then GW books are neither since they are not hand written or ancient. They are just books. However, it's just GW using the word in an 'atmospheric'...

You can still use it. Either is fine.

Not sure if English is your native tongue with your location, but either codexes or codices are considered acceptable pluralisations in English.
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