Oh man... I see them all the time on Tumblr, and people make master posts about it. I think a big thing is to know specifically what you want to improve, and to be OK with other people's art style. I think some people have big troubles learning new things cause they want to draw from an art style they look like when it's not as easy as that,

How to draw manga books- the original ones made in actual Japan are hit and miss. Some are pandery, and some are good. The deal is that to learn from how they teach, you need to do the exercises. Not just read them. They have a different teaching method to western taught books, If you're talking other How to draw Manga books, like ones that aren't Japanese translated, then those are dodgy. I like Mark Crilley's stuff, but he studied in Japan a lot. 

As for other things, there too much to list here. My latest fav is Proko's videos on Youtube. They taught me a LOT about life drawing and how to do it. I spent years doing night classes not getting it. IT all made sense when I understood what I was trying to do.