Dear Miss Snark:
If an agent doesn't keep a client informed on the status of a book and rarely answers infrequent emails, is it ever all right for a writer to contact editors? You may be wondering why I don't fire my agent, and believe me I'm thinking about it.
A big hello to KY from me.
If the book is sold to a specific editor at a publisher, yes.
If the book is on submission, no.
If your agent isn't answering your emails, get on the phone. If s/he isn't answering the phone, write a letter.
Remember though, publishing moves at a glacial speed and there simply may be nothing new to say.
That said, one of my biggest rants is agents who treat clients like they are a pain in the ass. Even if they are, they're the ones writing the stuff that brings in the money, and answering their emails is one of the requirements of the job.
KY wants you to know he offers agent contacting services at a discount rate to blog readers. First bite is free.