Found some really good advice for building up your visual library whilst reading Scott Robertsons "How to Draw" - Long story short, observe a specific subject e.g. Nose for a week, sketching from reference, then in the second week move onto sketching it from imagination with the aid of reference. Then move onto the next piece of the puzzle e.g. Mouth.
If you had enough time to practice, you'd be able to get a pretty thorough understanding of the whole body in around 12 ish weeks. Anyway, just thought it would was good to share! I'll be using it for my environment dev. focusing on a new style of architecture every 2 weeks!