You Need to Read a Lot

For various reasons:

• To know your market, both the classics and the current hits.  What is successful with the audience you are trying to reach?  What fails?  What has not been tried yet, and should be?  What has only been attempted poorly, and should now be tried again by a better writer... you?
• To see how other writers do it.  As you work out your own craft, see how other writers have solved the problems you face.  Look at big picture issues, like how many "acts" a book has and how dense its subplots are.  Look at small picture points, like how a writer tags his dialog, what POV a writer follows, etc.  Look at specific tasks and specialized issues, like flashbacks, dream sequences, drug-induced visions, and so forth.
• Knowing other authors' work will help you as a professional.  You will be able to talk about it intelligently with other industry professionals, e.g. your agent or editor, and if you know a writer's work, you will have a leg up when you bump into her at a convention and try to network.
• You need to have grist for the mill.  You should be reading non-fiction, news, comic books, fiction in genres you don't work in, whatever... it all goes into the sausage-making machine you call your brain, and with the right twisting and shaking, it can come back out again as tasty breakfast links.