The scrapbook would be a primary source.
Technically, the letters themselves are the primary source. The fact that they are contained together in a scrapbook is unimportant. They could just as easily be kept in a binder or folder.
A primary source provides direct evidence, such as the kind contained in these letters. A secondary source describes or comments on a primary source. The letters themselves are, then, a primary source. They would not be fictional or biographical.
