You don't have to buy it as you can borrow "Napoleon's Peninsular Marshals; a reassessment" by Richard Humble, 1975 via Internet Archive link
Other books on Napoleon's Marshals:
"Napoleon's Marshals" by Ronald Frederick Delderfield, Chilton Books, Philadelphia, 1966 also borrowable via Internet Archive link
"Napoleon's Marshals" by Peter Young, Hippocrene Books, New York 1973 also borrowable via Internet Archive link
"Napoleon's Marshals" by Richard Phillipson Dunn-Pattison, Brown & Company, Boston 1909 PDF link
"Napoleon and the Marshals of the Empire" by Rufus Wilmot Griswold, Lippincott, Philadelphia 1855 (2 volumes in one PDF link
"Napoleon's Marshals" edited by David Chandler, Cassell, London 2000 also borrowable via Internet Archive link
Macmillan, New York 1987 link
"Napoleon and his Marshals" by Joel Tyler Headley, C. Scribner's sons, New York 1888
Volume 1 PDF link
Volume 2 PDF link