Since there are many folks on this forum that are new to using Mathematica/WolframLanguage I'd suggest that each order a copy of Stephen Wolfram's soon-to-be-released (December 7 2015) "An Elementary Introduction to the Wolfram Language":
though I haven't read it yet, there is a clear need for a tutorial that helps beginning users "start right" and avoid the many issues that arise by simply diving in without a helping hand to get the basics in place.
Also another excellent book (but which assumes a certain amount of exposure to scientific, engineering and mathematical subjects but no knowledge of Mathematica and very little if any of programming in general) is Paul Wellin's "Programming with Mathematica®: An Introduction", which I have read.
So, beginners, get a couple of good introductions to Mathematica and dive in to learning the language!
I'd like to add that there are also two good online sources;
and the digital version of the book:
According to Amazon, there is a new version of Paul Wellin's book out in early 2016.
I would also add Cliff Hastings, Kelvin Mischo, and Michael Morrison's Book: Hands-on start to Wolfram Mathematica. There is a key in the back that gets neat stuff like a free on-line course ($50 value), so the book pays for itself.
I have seen Stephen's book in pre-release format, and started to read the on-line version. It takes a different approach from either these books, and is worth while just for the pedagogy.
These are all great additions to the list!
A good use for this thread is to post its link to beginners who ask questions but who we'd first like to suggest that they spend some time learning the language.
Here's the link:
I am not making it active as that'd lead to an infinite clicking loop ;-)