Gary Stonum discovered that "Roughly two hundred of Dickinson's poems include some reference to mathematical terms and ideas." Several authors (including Stonum) have discussed the impact of mathematics on Dickinson's poems. Independently, scholars like Richard Sewall, Cynthia Griffin Wolff, and Jack Capps have made the case that Dickinson was a serious and broad reader whose reading was indispensible for her poetic enterprise. This essay adds to that broadness of reading a genre heretofore not sufficiently linked to her poetry: her mathematics textbooks. Using a particular textbook—Jeremiah Day's Introduction to Algebra—we show how certain Dickinson poems reflect the specific ways Day talks about his subject. We make the analogy that Day can be for mathematical words what Webster's 1844 edition is for words in general.