That's tough, because now there are so many!
I'd start with a bunch of the free stuff: forums like CGTalk and Conceptart.org have good information and lots of inspirational art to study. Then there are a bunch of great tutorials and walkthroughs on youtube, from people like Feng Zhu, Tyler Edlin, Sycra, ConceptCookie, Dave Rapoza/Level Up, and more.
If you're willing to pay a few bucks, there are tutorials on Gumroad and Patreon from a number of artists...I don't have any links, but there are full tuts from Tyler Edlin and John Sweeney that look pretty good.
Then there's Gnomon, and their DVDs. Pricey, but very educational.
Then there are various "master classes" offered by Imaginism studios,
www.cdwstudios.com/, CubeBrush, and others.
Some of the "making of" books for various films and games show a lot of good production art that can be educational to examine. Some of the ones on my shelf are "Art of Star Wars", "Art of The Hobbit", "Art of Halo 2", "Art of John Berkey (awesome 60s sci fi paintings), and others.
I don't know much about character design, because I focus on environments, but I'm sure there's a ton of free stuff out there on DA and youtube that you could start with.
Good luck!