[davidsiems]

If you've never read it before this book is a very intuitive introduction (and refresher): http://www.amazon.com/Calculus-Made-Easy-Silvanus-Thompson/d...

[ashconnor]

This book is currently out of copyright (published 1910):

http://djm.cc/library/Calculus_Made_Easy_Thompson.pdf

[Semiapies]

Also available at the Gutenberg project, here: http://www.gutenberg.org/ebooks/33283

It seems to be a rather nifty book.