Everyone has “sources” they depend on to write well. I rely heavily on the Mayo Clinic’s website (for useful health references) and Rhapsody of Fire blasted on MAX (for inspiration) to help me get started. For others, Tagcrowd or Textalyser helps keep their articles SEO-rich and focused. When I’m deep into my article, though, I’ll use Thesaurus.com and the sources listed below to flesh it out.
If you really want to improve your writing, these three sources are a must. It won’t make you a writing god, but …

Authored by one of Associated Content’s most prolific contributors, Sabah Karimi, The AC Source Book walks you through the basics of publishing–and succeeding–with Associated Content.  Although the book is geared towards new contributors, even experienced writers can learn from this book, which includes effective promotion methods, how to specifically use search engine optimization (SEO) in articles, and how to create articles that garner substantial page views.
Specifics
The AC Source Book is a walkthrough through the entire Associated Content submission process.  It includes information found by participating in the AC forums, but …

Common Ties
Want to release your innermost thoughts and feelings to possibly secure a book deal?  Then Common Ties might be for you.
Who They Are: Common Ties (commonties.com), is looking for illustrated submissions to insert in their upcoming book, set for release in 2010.
What They Want: Common Ties is looking for illustrated submission answering one of their twenty questions on the website.  The answer must be 50 words or less — but this shouldn’t be a problem if you’re already accustomed to twittering.
When They Want It: Common Ties is accepting submissions …