Books on Writing
There are a million of them out there and what was useful to me might not be for others. But these three helped me the most to improve my writing:
The Art of Fiction by John Gardner
The whole book is helpful, but Gardner has one chapter entitled "Common Errors." I read it and realized I was committing most of them.
Characters and Viewpoint by Orson Scott Card
It's one thing to have a story in your head, another to tell it in a compelling way. Card shows example after example of how to bring characters to life and create tension and resolution.
Writing on Both Sides of the Brain by Henriette A. Klauser
Klauser give techniques that enable the writer to tame to the inner critic that stifles the early creative process. More than any other single tool, it helped me understand and conquer writer's block.