I really do think that Loomis' old school figure and head drawing books are really excellent. It all depends a bit on what you'd like to do.
If you're interested in the human figure and drawing from imagination, Loomis is a really good place to start, so I would take time with him, he's worth it. I would also recommend Glen Vilppu's figure drawing book and his old school video course, he has great advice on imagination drawing. That was one of the things that got me started. If you're interested in very realistic observational work, then you might have to complement with some other sources, like Tony Ryder's guide to figure drawing, Solomon J. Solomon's book and Harold Speed's "Practice and science of drawing". The very conceptual approach (where everything is based on imaginary 3d forms like cubes, spheres, etc) of the first books can lead to some mannerisms, if what you're interested in is hard realism. These last ones have more advice on drawing from observing the model very accurately and things like tone and rendering, which are necessary for realism. But again, it depends a bit on what interests you the most (not that you can't blend and practice both, of course).
If you're interested in the human figure, anatomy can be pretty important. My single favorite anatomy book is "Artistic Anatomy" by Paul Richer. This guy set the standard for anatomical knowledge as it concerns artists back in the XIX century. A large amount of modern anatomy books (including pretty much every single anatomy plate in Loomis) are based on his original work. The only downside is that it contains almost no drawing advice and is very technical, so it might be worth waiting a bit before approaching it. There is a German author, Gottfried Bammes, who has some interesting books on artistic anatomy with some more conceptual plates, esp "Die Gestalt des Menschen" that are pretty good, as well as an Italian book that is based on the same approach called "Struttura Uomo". Those last 2 books blend anatomy and conceptual figure drawing pretty nicely (no need to bother with the text). I would also recommend getting a skull and studying it from different angles if you can get a hold of one (I think they're like $20-$50 on amazon). And if you get the chance of getting people to pose for you for portraits (or if you can get them nude, even better lol), that will be great practice as well.
If you're more interested in objects and perspective, I think Scott Robertson has amazing stuff. A bunch of dvds and some books. He's great at creating vehicles, objects, spaceships, etc in believable imaginary 3d space and renders them from very successfully. If you're interested in creating environments with a bunch of stuff in them from imagination more than in the figure or portrait, that will be helpful, as well as some perspective books. But at this kind of stuff I'm less experienced.
I think Proko has good stuff, I haven't really delved into his website, I met the guy very briefly once haha. I've seen some of his advice online and I think he interviews a lot of great artists, so he seems interesting.
I know that is a lot, but drawing is very broad and has a bunch of facets, kinda like martial arts. You might want to specialize or do all of it, so your approach will depend on what interests you the most.