Books that are histories/text books of fictional worlds?

I can't think of a good way to describe this, so here are some examples:

• World War Z
• Star Wars: The Rise and Fall of the Galactic Empire
• Many of the old Star Wars legends guides (ie The Essential Guide to Warfare)
• The epigraphs in books like American War or many Warhammer books (Ashes of the Imperium is what comes to mind)

Basically something that someone in that universe would also read