I cannot stress how invaluable the Web has been in doing research on the publishing industry, agents, querying, and just about everything else. Very recently The Bookshelf Muse did a blog roll asking readers to submit what blogs they can't live without. The list in full can be found here and is a wonderful "cheat sheet" on where to get started if you're just entering the game or don't follow these blogs yet.
A few blogs and Websites, in particular, have been invaluable. On understanding the industry and current trends I find Kristin Nelson's Pub Rants and Nathan Bransford's blog a huge help. On researching agents, Casey McCormick's Literary Rambles, Guide to Literary Agents, and, of course, Publisher's Marketplace.*
Regarding queries, I suggest everyone stop by Janet Reid's Query Shark and study the advice she gives and for the truly bold, I suggest submitting your query along. It may mean public embarassment but also a free, professional critique from a superstar agent and, if you're lucky, maybe even representation. Query Tracker blog is also really helpful. I have to admit though I read Query Tracker more for the articles from guest agents and writers than to track my queries. In my defense, I already had a query tracking system in place based on some advice Jeff Kleinman of Folio Literary Agency gave at a conference.
And the blogs I read each morning between the Washington Post and my second cup of coffee (because they're either really helpful, really fun, or both) are Suzie Townsend's Confessions of a Wandering Heart, Kathleen Ortiz's Neverending Page Turner, Lisa and Laura Roecker's Lisa and Laura Write, and Sarah Wylie's Sarah With a Chance.
More recently, I've also been checking in with Adventures in Children's Publishing for helpful links and some amazing contest because, really, who doesn't love free goodies?!
Perhaps the best piece of advice, however, is if an agency or an agent has a blog or has done some interviews that you can get your hands on, READ them. This is the holy grail. You get a really great sense of agents' personalities, what kind of agent-author relationship they try to garner, and what specifically they're looking for both in a query letter and in a book.
Do you have any helpful blogs or Websites I didn't list here?
*Publisher's Marketplace does a free "Publisher's Lunch." For $20 a month you can also purchase a subscription. Trust me, it's worth it. Through the subscription you can look up author representation, agents listed by genre, and see who is selling what plus much, much more.