http://jetreidliterary.blogspot.com is a blog by Janet Reid. A literary agent, she blogs very frequently about various writing-related topics.
Amongst the resources on her blog are her query letter checklist, query letter FAQ, and query letter diagnostics (why you didn't hear back!).
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Janet Reid, Literary Agent
A blog by the literary agent Janet Reid, on writing-related topics: jetreidliterary.blogspot.com