Where are the best resources for learning how to create Drupal Themes from a visual design?
I'm looking for online tutorials or books, which will give me the best understanding in a practical way.
I don't think there are any resources specific to creating Drupal themes from visual designs, but a Drupal theme is after all just HTML+CSS with a sprinkling of PHP, really any resource that helps slicing up visual designs into HTML and CSS should be a help.
For the Drupal side of things I'd say the best thing is just to dive in and try, you'll learn better by doing than trying to follow tutorials, even though you may lose some hair in the process. Choose a side project to start with, not something with a looming deadline!
Drupal Dojo has some lessons on theming
Packt do a whole range, including several relevant to themes
Training, if you need hands on help:
Most of all you'll need determination and patience. Good luck.
There're some really useful Drupal video podcasts here: