It seems like he had two books. ABE books has a copy of a book by him called Genealogy of Clan Donald that is 461 pages rather than the 66 of the other one. There's also an article from 2007 on the Global Genealogy site that says:

Duncan MacDonald, the founder of MacDonald Research Centre in Brockville, Ontario, spent a lifetime transcribing and publishing the church records and family histories of the settlers and inhabitants of Stormont, Dundas and Glengarry Counties in Ontario, Canada. There is some overflow into neighboring counties and adjacent areas of Quebec and USA. Mr. MacDonald's considerable efforts effort resulted in eighty nine published books.

At the beginning of this month, Global Genealogy's publishing arm Global Heritage Press Inc., completed an agreement with Mr. MacDonald's heirs, to acquire all publishing rights. We also assumed a considerable inventory of unsold new books that remain.