Here's a handy link for all designers and for designer’s clients http://www.aiga.org/content.cfm/design-business-and-ethics Run by AIGA – American Institute of Graphic Arts, you'll find various pdfs to download covering topics such as: Client’s guide to design, Use of fonts, Use of illustrations, Use of software, Sales tax, Guide to copyright, Use of photography. It's a pretty comprehensive list, there's very little that isn't covered here.

Another book that overlaps on a good few of the above is: http://www.amazon.co.uk/Graphic-Designer-Without-Losing-Your/dp/1856694100 and another: http://www.amazon.co.uk/Its-Not-How-Good-Want/dp/0714843377/ref=pd_sim_b_2

Pic above is taken from this hilarious blog: http://thereifixedit.com

I was lucky enough to see George Lois speak at a design conference called Offset last week. I was familiar with some of his work (Esquire mainly) but not all of what he presented on the day. The man is a serious inspiration to all designers out there. Ideas. Ideas. Ideas. He repetedly stressed the importance of ideas in graphic communication. No cheap tricks using flash-in-the-pan styles, make sure all graphic solutions have a heavy weight idea at their core. Proper order. Too much shite, trendy design out there these days with no substance in my opinion. I am itching to get my hands on his new book. I'd say it's an impressive read, similar to Tibor Kalman's book or Sagmeisters.
His site: http://www.georgelois.com/  where you will see much of his work and you'll find a link to his book. One other interesting point I never knew, the man's mentor was Paul Rand. Right place, right time!