While I am not familiar with the qujotation "Truth is more a stranger than fiction," Twain penned two different maxims closely related, both which appear at the beginning of Chaper XV in Following the Equator from "Pudd'nhead Wilson's New Calendar. You cited the first, and better known. The other reads "Truth is stranger than fiction--to some people, but I am meansurably familiar with it." The original source for this particular qote is from Twain's Notebooks, #33, p. 59. Joe McCullough