I remember my childish puzzlement at a photograph, in American Heritage or some such publication, of Teddy Roosevelt and the Rough Riders at San Juan Hill. The caption described him giving his famous order — “Gentlemen, charge!” — in a “high falsetto.”

Surely American Heritage was mistaken. Even if you know it, it’s hard to remember that the man whose muscular credo was to “speak softly and carry a big stick” and who is most often imagined as an extreme adventurer was in youth pampered and sickly.

In his engaging “Becoming Teddy Roosevelt: How a Maine Guide Inspired America’s 26th President,” Andrew Vietze makes a persuasive case that the transformation happened in the woods of Maine, and that the catalyst was a Maine hunter and woodsman, William Wingate Sewall, some 14 years the future president’s senior.

Vietze’s narrative begins with the teenage Roosevelt arriving in Island Falls, Sewall’s home, “wearing big spectacles and weighing a scant 135 pounds.”

Two well-wishing cousins had brought him with them. They had enjoyed Sewall’s guiding prowess on previous occasions, and thought it would be just the thing to toughen up young Theodore. Roosevelt was not only a weakling, but he was asthmatic and had a bad heart too, so they warned Sewall to take him under his wing but not push him too far.

What happened over the next two weeks is memorably told. Vietze’s own experience and knowledge of the Maine Woods — he is a ranger at Baxter State Park as well as a writer — help him write with special empathy about these expeditions. TR, as the author tends to call him throughout, made two further trips to northern Maine, one the following winter and another the next summer. On the second, he reached the top of Mt. Katahdin, which neither of his cousins managed to do.

“Rather to my surprise, I found I could carry heavier loads and travel farther and faster than either of them,” he recalled. And, Vietze writes, he marveled “at the fact that he could keep up with the Mainers.”

Not only did these adventures make the man; they were the foundation of a life-long friendship. On many occasions, Roosevelt professed the yen to return to the world of Island Falls. He never made it, but the two men never lost touch with each other.

Five years later, TR persuaded Sewall to bring his family out west to the Dakota Territory to manage his cattle ranch. The Mainers seem to have mostly hated the dry and barren West, but they put a good face on it.

Vietze gives a splendid account of their adventures there, which lasted two years. After that, as the cowpuncher turned politician, “(t)heir friendship, once built on physical pursuits, became an epistolary one — letters to ‘Friend William’ made a steady march north over the decades.”

Vietze makes clear that throughout his life, Roosevelt had a special place in his heart for his erstwhile mentor. On a visit to Bangor, Roosevelt — president now, after McKinley’s assassination — started his speech asking, “Is Bill Sewall in town? Has anyone seen him? Tell him I’ll be disappointed if he doesn’t come.” And when he was elected president in his own right, the Sewalls were guests at his inauguration.

Against the backdrop of Roosevelt’s historic career, Vietze brings to life the town of Island Falls — Bill Sewall was the first white child born there — and its travails. Thanks to their correspondence, he is able to weave the lives of the two men together so that the reader is always gratified when they meet. Sewall made several visits to the Roosevelt White House, and evidently cut quite a figure in the press.

The one controversy in Sewall’s life appears to have been when he sought and received the president’s backing in his campaign to become customs collector for Aroostook. Among the book’s excellent illustrations is a newspaper clipping from the time that “cast it in a nepotistic light.” Not that the book needs controversy, but in view of the evident love and admiration Vietze developed for Sewall, it is a pity that he brushes over it, merely referring to it as a “touchy subject” mentioned by “Maine humorist S.A.D. Smith.”

In “Becoming Teddy Roosevelt,” Vietze celebrates the extraordinary qualities of two men: the one largely unsung, the other well-known, larger than life, but rarely shown in so intimate and human a light. It is a joy to read.

Thomas Urquhart is the former director of Maine Audubon and author of “For the Beauty of the Earth.”