doing

/data get @s Inventory[0].components.minecraft:writable_book_content.pages[0]

 on a writable book will return {raw: "text"} so using this in something like a macro will give text but using

/data get @s Inventory[0].components.minecraft:written_book_content.pages[0]

 on a written book returns {raw: '"text"'} so in a macro it will give "text" side by side that's

text , "text"

the written book has extra quotes witch makes it imposable to convert between them or run functions from a written book as you can not use quotes in a function name