One of my favourites is Hiroyuki Kojima's "Manga Guide to Calculus" (http://nostarch.com/mg_calculus.htm http://nostarch.com/mg_calculus.htm). It probably can't

One of my favourites is Hiroyuki Kojima's "Manga Guide to Calculus" (http://nostarch.com/mg_calculus.htm http://nostarch.com/mg_calculus.htm). It probably can't get much more informal and it's actually quite entertaining. It somehow manages a decent balance of being fun without slipping into being trivial.