[snprbob86]

Start with Mathematica's builtin documentation. The overview pages are actually quite good. They are also online [1].

Of course, given that the early drafts were written by Wolfram himself, you'll have to ignore absurd statements like this one: "Long viewed as an important theoretical idea, functional programming finally became truly convenient and practical with the introduction of Mathematica's symbolic language." [2]

See also: Pure [3]

[1] http://reference.wolfram.com/mathematica/tutorial/CoreLangua...

[2] http://reference.wolfram.com/mathematica/guide/FunctionalPro...

[3] http://purelang.bitbucket.org/