What to put in the source title field

The following query and response passed back and fore by email, but it's worth posting them here in case anyone else is wondering the same thing:

Query: Does the Title of a source book *ever* go in that first source field or is it always in the format of 'Letter from X to Y, date xxxx', 'Diary entry by Y, date xxxx', regardless of whether or not the material has been published in book form?

Response: If it's an individual diary-type entry (or letter) - i.e., something that can be identified as a discrete item - use a description such as 'Memoir of Ada Galsworthy, 2 November 1878' in the 'Source' field on the main form, and then tick 'published' and 'as part of' in the source sub-form and fill in the publication details. If it's a continuous narrative that doesn't break down into neatly identifiable chunks, use the book title in the field on the main form.