The problem with referencing anatomy is that though you may be able to find a good reference in the pose you want, all that will do is help you draw the pose you want. Sounds exactly like what you want, I know, but when you consider that you're almost definitely going to draw someone in a different pose with different proportions in the near future... well, learning how to draw the outline of the one pose won't have helped you much.
I myself have no idea what's the "best" way to learn anatomy. You might think it would be to get one of those hefty books with all the muscles and bones and muscle names, because, hey, they're "anatomy", right? Well, though you will have to learn muscles and bones eventually, I believe you'd be better suited to learning the basic structure of the body and how to make a pose look like it's doing what it's doing before you worry much about how the deltoid and bicep interact.
Quite a few people I know swear by the book "Force : Dynamic Life Drawing for Animators" by Mike Mattesi. It's a fair building block to start from, but it doesn't really go as much into structure as you should (something that I'm still fighting with, myself), though it DOES push a very fun and exciting/dramatic way to draw. Rhythm, man. If you capture the rhythm of the pose, it'll look cool no matter how much you know about muscles (unless you bomb your proportions, but that's another story).
Force book is great, but as mentioned, it lacks structure. A book that fills that hole (so I've heard, don't own it myself) Would be the Vilppu Drawing Manual. Vilppu is one of the old guys who worked for Disney and as such he literally wrote the book on drawing. I've only watched the instructional DVDs (some of them) and though he draws slow as all hell (so you can actually SEE what he's doing, I assume) he draws virtually flawlessly and helps greatly to understand the process and how to simplify forms into geometric shapes.
The third recomendation that I've got is a touch expensive for its size, and I'm sure there are things similar elsewhere, but I'll mention it because it's the only one I know. Wayne Gilbert's "Simplified Drawing for Planning Animation" brings you back out of realistically-proportioned drawings and into creating cartoony characters. It explains further simplification and how to design a silhouette for your character that tells the viewer what your character is about, just from the silhouette.
Other than that, draw. Once you understand basic simplification somewhat, go sit in a coffee shop and covertly draw people. Like a ninja. Try not to laugh too hard when you draw someone funny-looking, though, because people tend to think you're crazy pretty quick :/
In case you're too lazy to hunt these books down yourself:
Simplified drawing for planning animation, Wayne Gilbert:
http://www.amazon.com/Simplified-drawing-planning-animation-Gilbert/dp/0971343918Vilppu Drawing Manual, Glenn Vilppu:
http://www.amazon.com/Vilppu-Drawing-Manual-Glenn/dp/1892053039Force: Dynamic Life Drawing for Animators: Mike Mattesi
http://www.amazon.com/Force-Dynamic-Drawing-Animators-Second/dp/0240808452I'm not saying these are the best books money can buy and that they'll make you a great artist in 6 months (Though you can improve a hell of a lot in 6 months, believe me
) but they'll give you a good building block to start from, and aren't that hefty an investment. An' I really can't blame you if you don't even consider getting any of these books at all; I'd have done the same a year or two ago. I do recommend getting the Force book, though. It's fun.
That's all! Cheers.