I love introductory textbooks. Some of my favorites:

"Starting Forth" - Leo Brodie

"Computer Science, Logo-Style" series - Brian Harvey

"Algebra", "Functions and Graphs", "Methods of Coordinates", "Trigonometry" - I.M. Gelfand (does anyone know if he completed his Geometry textbook? I've never seen it. I've also never seen his "Calculus" or "Combinatorics" books, although I know they exist. I suspect his "Combinatorics" book is more advanced than he intended to produce.)

"Sage for Undergraduates" - Gregory Bard