Writer's Digest has been the bible of freelance writing for a long time. I started reading it in college, and I devoured every article in every issue. I also sent away for some of the books put out by their publishing company, and I thought they were useful, to varying degrees.
Art Spikol, an ex-boss of mine, wrote a column for Writer's Digest for years. Art's column, like the magazine, was full of practical advice for freelance writers.
I don't subscribe to the Writer's Digest magazine anymore, but I do visit their Web site. And this particular page, The 101 Best Websites For Writers, is one I have bookmarked. It's got some very useful resources in categories like Agents, Jobs & Markets, Writing Communities, and Fun for Writers (every writer needs to have some fun!). It's worth checking out this site.