Both Diaries and journals are called, "Blank-books" in Ellfa. I drew a 'Redh' edition here.
Each blank-book is only accessible by the allowed persons set by the original buyer. For example if Elijah originally has it and allows John to write in it, only they can successfully read/write in the book. Anyone else who tries to will only see blank pages, and all writing made by the them will magically disappear.
After the writer(s) death all pages will be locked unless writers say otherwise. -Az