I think the best way to improve your drawing is purely practice...but for me those books helped me soooo much!! they showed me how to get the body and face in proportion as well as how to make the bodies look more natural...here is a fan art thread i did, check it out if you want some proof that these books can work...i'm still an ameture drawer but this is the result of those books (in my case anyway). If you want to beable to make up your own characters i've got some on on page 2 that i did myself as well.
FAN ART!! here goes :|
the only annoying thing is those books is that they're kinda expensive...try looking at how other people draw (a grerat help is to also study the pictures in the manga's you read) and also life drawing will give you a great advantage. try posting some of your stuff up, people will give you help, that way so that you know what to work on for next time; use criticism to your advantage!
i hope this helps! (also i think just getting one book is enough, no need getting all of em. once you've got the basics it would just be a waste of money to buy more. if you were stuck on a particular image that isn't explained in the beginners book just flick through a manga to help you out or if you can't get a pose right just look in the mirror, or get someone to model)