You will need to max out the notebook (open notebook) as well as the fountain pen. Once you have both, tap the open notebook, and the pen will merge in to create a book. You will be able to create several books with more pens from one open notebook

I kept trying to merge them but no luck..many random attempts finally figured out it was just a tap to the book lol. Same process a little later down the line with the sewing machine

Once you have the max notebook (leather open pages) and the fountain pen tap the notebook and it will suck the pen towards it and turn into a book