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What do you mean a different medium? Are you talking about those big old timestamped books of references that used to come with anime? Or an actual in-universe book, such as The Elder Scrolls In Universe Books?
I means something like what Mary Gentle does with her Ash series: clips that could be email inserts or cut from a print-screen image. Inter-spaced with actual block text. Framing Device is dead close. Very close, in fact, as it could be stretched to mean this.
Edited by Euodiachloris

I'm looking for a trope that's somewhere between Translation Convention and an Infodump - where a missive and/or insert from another medium is included within either the narrative or main body of text. Does it already exist?