Calculus

Beginner

Calculus Made Easy Silvanus P. Thompson

Easy

Calculus James Stewart Thomas' Calculus Hass, Heil & Weir

Medium

Calculus with Analytic Geometry George F. Simmons

Hard

Calculus Michael Spivak

Extreme

Calculus, Vol. I Tom M. Apostol Calculus, Vol. II Tom M. Apostol