I can't remember when I first discovered this blog but it's one I'm enjoying more and more each week. It's called The Rejecter and it's written by an assistant at a literary agency. As she notes (and yes, the author is a "she", at least according to the blog's profile), "I am the first line of defense for my boss. On average, I reject 95% of the letters immediately and put the other 5% in the "maybe" pile." The blog's subtitle is "I don't hate you. I just hate your query letter." Gotta love the attitude!
I'm a big fan of sarcasm but you might be put off by the snarky comments. Don't be. This blog is loaded with great info. For example, here's an excellent post covering the financials from the agency side of the business. If you've always wanted to take a peek at the world from an agent's point of view, this is the blog for you.