I read this as in connection with Defoe's The Storm (the two works were combined in one volume). You can read my review here. I was confused about whether The Storm was really a work of fiction or not, and Minto really helped to put it in its historical context as an example of the journalistic standards of the day.
I'm afraid that all but the most serious Defoe nerds will be disappointed with this one. It gives some valuable historical contextualization and there are some golden moments where Minto shines, such as the last few pages, but all in all, it's tedious and doesn't give a very satisfying or thorough portrait of Defoe.