Notebook to write your feelings, thoughts, etc. about a person (deceased) or an organization on a special occasion
Book of Remembrance
A ‘book of remembrance’ is a special book to commemorate someone.
The French have a term for ‘visitors book’ which is ‘livre d’or’ or ‘golden book’, which is a nice idea as well.
https://www.veterans.gc.ca/eng/remembrance/memorials/books
If it's about a deceased person, it's a book of condolence.