Previously an unread written_book had "resolved:0b" parameter that was searchable with commands, now it no longer shows with a data check, even if the give command stated it.
Resolved:1b is searchable with command after book is resolved though.

/give @p minecraft:written_book[minecraft:written_book_content={pages:['[{"text":"Page1, "},{"selector":"@s","color":"red"}]'],title:'Anything',author:'Blank',generation:2,resolved:0b}]

While holding unopened book using, /data get, you'll be unable to find "resolved:0b"

/data get entity @p SelectedItem

Open the book, repeat the data get command and you'll see it now displays the "resolved:1b".

Another command to prove the point, just change the following {resolved:1b} to "0b" to see that before reading the same search is not possible using the new components system.

/execute if entity @a[nbt={SelectedItem:{components:{"minecraft:written_book_content":{resolved:1b}}}}] run say toot