Looking for advice on learning GP-Pari

[Daniel]

Several years ago Stephen Wolfram offered to give me a copy of Mathematica. Unfortunately the gift was only for one year. A few times I reached out to Wolfram Research to renew Mathematica. Wolfram asked me to write Mathematica software for Wolfram Functions supporting tetration, but I have to either buy or beg for Mathematica. So time to move on to a public domain mathematical programming language. 

What resources do folks recommend for learning GP-Pari? I looked on Amazon and there is only a couple of expensive books on using the programming language. Is it all just online resources? 
Thanks,
Daniel