I think my experience in 2d drawing helped me a lot in sculpting. Unfortunately I don’t have any resources for hairs beyond google image/pinterest/tumblr :( For more general stuffs I learned a lot from Andrew Loomis books http://pl.scribd.com/collections/3106877/Art-Books :D
I self-taught almost everything :) Zbrush is pretty easy, there’s a lot video tutorials on youtube for anything you wish to know. I can’t suggest you anything specific because I just watch only this for specific problems. But I read somewhere this book http://www.amazon.co.uk/ZBrush-Digital-Sculpting-Human-Anatomy/dp/0470450266/ref=la_B001JSASIQ_1_1?ie=UTF8&qid=1368816851&sr=1-1 by Scott Spencer is pretty good. I will buy it myself probably :)
I started to sculpt in Zbrush, first time for serious purpose :) I have few ideas for dota2 head items
http://steamcommunity.com/workshop/filedetails/?id=143513535 PA Danse Macabre set published on Dota Workshop. You will find more description on workshop page :)

- Get out of here at once, this is my trailer!
- Sure, sure chump, it’s your trailer only I’m the resident ghost, see. I have full rights to occupy this place.

Silencer DOTA 2 set concept
*idea*
If he was raised by mages how did he know how to fight? Mocked by his kind maybe he was closer to barrens natives, and learned combat skills from elder warriors. His armor decorated with primitive symbols of physical strength characteristic for savages aroused disgust in other mages but brought him champion title after all.