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.

Helen