Daniel Defoe is an exception to this rule. He was a man of action as well as a man of letters. The writing of the books which have given him immortality was little more than an accident in his career, a comparatively trifling and casual item in the total expenditure of his many-sided energy. He was nearly sixty when he wrote Robinson Crusoe. Before that event he had been a rebel, a merchant, a manufacturer, a writer of popular satires in verse, a bankrupt; had acted as secretary to a public commission, been employed in secret services by five successive Administrations, written innumerable pamphlets, and edited more than one newspaper. He had led, in fact, as adventurous a life as any of his own heroes, and had met quickly succeeding difficulties with equally ready and fertile ingenuity.
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.