The Lion of the North, A Tale of the Times of Gustavus Adolphus by G. A. Henty

Historical novel set during the Thirty Years War. According to the Preface: You are nowadays called upon to acquire so great a mass of learning and information in the period of life between the ages of twelve and eighteen that it is not surprising that but little time can be spared for the study of the history of foreign nations. Most lads are, therefore, lamentably ignorant of the leading events of even the most important epochs of Continental history, although, as many of these events have exercised a marked influence upon the existing state of affairs in Europe, a knowledge of them is far more useful, and, it may be said, far more interesting than that of the comparatively petty affairs of Athens, Sparta, Corinth, and Thebes. According to Wikipedia: George Alfred Henty (8 December 1832 - 16 November 1902), referred to as G. A. Henty, was a prolific English novelist, special correspondent, and Imperialist born in Trumpington, Cambridgeshire, England. He is best known for his historical adventure stories that were popular in the late 19th century. His works include Out on the Pampas (1871), The Young Buglers (1880), With Clive in India (1884) and Wulf the Saxon (1895).