Most writers can benefit in all sorts of ways from having a good agent: agents make sure that their clients' work gets seen by the very best publishers, and they negotiate contracts which squeeze out every last drop of value for their clients. I've written several blog posts about agents, and am certain to write more.
Many agents have websites and blogs, and here are links to several. This is by no means an exhaustive list as the Writer's Handbook (and other similar guides) already do that so much better than I could. These are the places that I find the most readable, informative and fun. Do please let me know if any of the links are broken, so that I can correct them; and if you know of a useful and informative agent website or blog, do please link to it in the comments so that we can keep this list as up-to-date as possible.
Andrew Lownie is a London-based literary agent with a preference for non-fiction, and his website has a good list of articles and opinion-pieces of interest to writers. Subscribe to Andrew's newsletter: it's consistently informative and funny.
Simon Trewin heads up United Agents and has a very useful blog.
Jenny Bent, of The Bent Agency, has a new blog with a few very useful posts already in place.
BookEnds, LLC is a New Jersey-based literary agency with a blog written by Jessica Faust.
Nathan Bransford blogs frequently and is very well-regarded. He's an agent with Curtis Brown in San Francisco.
Dystel and Goderich is an agency in New York, with an interesting blog which all of its agents seem to contribute to.
Colleen Lindsay is an agent at Fine Print Literary Management and her blog, The Swivet, is on my list of essential daily reading.
Pub Rants: a blog from Kristin Nelson of the Nelson Literary Agency in Denver. Make sure you subscribe to her agency's newsletter, which is always good.
Rants and Ramblings is the blog of Rachelle Gardner, a literary agent at the WordServe Literary Group. WordServe represents Christian writers, and works out of Colorado.
Janet Reid is based in New York. She has her own blog which is updated almost daily, her own website, and works at Fine Print Literary Management.
The Rejecter: a blog written by an anonymous assistant at a literary agency.
Miss Snark was a much-loved, but incognito, New York-based literary agent who stopped blogging in 2007. Her blog remains available, and is well worth reading. And no, I don't know who she was.