Lauren Ruth is a full-time literary assistant at Bookends LLC. She maintains a wonderful blog, called Slush Pile Tales, where she analyzes query letters for what the authors did right, and what turned her off. Reading her comments will make your query letters stronger.
I like how Ruth gives readers an opportunity for feedback on many of the query letters. She often ends the post with a one-question survey, so readers can share their own opinion. These query critiques are called Query Dice, and often answer questions writers have about what to include in query letters:
Query Dice 1: What if there are too many things happening in the beginning of the novel? What if the novel is too short?
Query Dice 2: What will an agent think if you leave out the salutation? Will typos earn a form rejection immediately? She also shares the need to include strong conflict in the query.
Query Dice 3: How writing credentials can make up for a less-than-stellar query.
Query Dice 4: What is the best salutation for a query? This query shows how small prepositions used closely together can make your writing look weak.
Query Dice 5: Is it ok to begin a query with a question? If it's a children's book, how specific should you be with the age-range of the reader?
Query Dice 6: Should you use ellipses or em dashes in a query? Is it ok to describe your novel by naming similar published books?
Other interesting posts:
If You Build It They Will Come: finding your voice
State of the Inbox Address: It's interesting to see what can be learned from an agent's statistics.
One Author's Journey: One of Ruth's clients shares her path to publication through self-publishing.
Have you written a query letter? How do you think it would fare under Ruth's scrutiny?