I'm going to nominate:
The Virginian by Owen Wister. I've been wanting to read a Western or two for some time and this seems like a reasonable place to start.
Spoiler:
In the untamed West, pioneers came to test their fortunes -- and their wills. The Wyoming territory was a harsh, unforgiving land, with its own unwritten code of honor by which men lived and died. Into this rough landscape rides the Virginian, a solitary man whose unbending will is his only guide through life. The Virginian's unwavering beliefs in right and wrong are soon tested as he tries to prove his love for a woman who cannot accept his sense of justice; at the same time, a betrayal by his most trusted friend forces him to fight against the corruption that rules the land. Still as exciting and meaningful as it was when first published one hundred years ago, Owen Wister's epic tale of a man caught between his love for a woman and his quest for justice exemplifies one of the most significant and enduring themes in all of American literature. With remarkable character depth and vivid passages, "The Virginian" stands not only as the first great novel of American Western literature, but as a testament to the eternal struggle between good and evil in humanity. With an engaging new introduction by Gary Scharnhorst, professor of English at the University of New Mexico, this volume is an indispensable addition to the library of American Western literature.