The letters to which you have access here represent part of the correspondence among members of the Gagnon family. They survive because they were addressed to a prisoner in a federal penitentiary in connection with her requests for release. This is the reason why we find them in the "Capital Case File" of Marie-Anne Houde, at the National Archives of Canada.
The letters in this series were written at different periods, but generally all had the same goal: to give and to ask for news. They mention news from home and progress at school, but especially the desire to see the prisoner again and the hope that she would return home. The letters were handwritten by the people who sent them. They were written on both sides of ordinary paper (resembling foolscap) and were folded. The letters were not all dated, but they were all signed. The level of writing wasn't very high and there were numerous spelling mistakes, suggesting a rudimentary level of education.
More generally, private correspondence constitutes one of the most useful types of sources in social history. Unlike official documents, private letters are not systematically kept in archival collections, unless they are part of the private fonds of a relatively well-known figure. The personal letters written by "ordinary" people can nevertheless be of great use for those who want to understand the nature and the content of friendly, family, and even romantic relations of a bygone era.