Non-writable books are scripted books, journals, etc, that make it appear as if a character is reading or writing in a book, but unlike a writable book, no actual text on a page is produced. A quill or other writing instrument must also be held if the character is to appear to be writing.
You analyze the poetry journal and sense that the item is largely free from merchant alteration restrictions, but the creator has stated that "The item has restrictions on changing the noun." The creator has also provided the following information: You get a strong sense that the journal can be altered by any merchant, but the noun must be one of the following: book, tome, volume, ledger, journal, folio, spellbook, grimoire, prayerbook, scroll, or vellum. The way the journal looks should make sense when it is both opened and closed. Some merchants may be willing to put any writing you have on your journal into a specific language, so that only those who understand that language can read the writing. Currently you are able to use WRITE, OPEN, CLOSE, TURN, and READ with the poetry journal. For writing, you will need to be holding a quill, though some paintsticks may also work for this. You get no sense of whether or not the journal may be further lightened.