The Cossacks by Albert Seaton (Osprey 'Men-at-Arms' series), paperback, 1972.
Bought this book on E-Bay to help me paint up 28mm scale Renaissance/Early 17th Century Cossacks for forthcoming games with friends using 'The Pikeman's Lament' rules. Such a great book despite being out of print. Excellent colour plates and informative historical text.