An anon asked a question in the comments of my Submission Follow-up Post: In case of an offer of rep, should an author send emails to any agents they have queried or merely the agents who have requested a partial or full?
At the end of 2012, when I first ventured into my boss's slush email, she had emails from the beginning of 2012 that still needed to be processed. My organizational soul was positively shocked, but my boss's priority is, and always will be, her current clients. The other interns had things like "lives" and "families" occupying their time, while I was pretty unencumbered by such matters. So I was the logical choice to start imposing order onto chaos.
The first thing I did when I opened a query was to search for any other emails associated with the queryer's email account. In some cases, my boss had already made a decision about the query, but neglected to file the query email in the appropriate folder. In others, the query might have been marked as unread in the inbox, but the full manuscript was sitting in the aptly named 'fulls' folder. And sometimes there would be an email letting my boss know that she'd missed out and the queryer had been signed by someone else. Those were the easy ones because the next step after the email search was to Google the query's author (a post about the importance of having an online presence will go live on 1/13/13).
I Googled pretty much every query I read because I was looking to see if the book had been self-pubbed or if there was agent information listed anywhere on the author's website. If the answer to either of these questions was yes, it meant we didn't have to spend time evaluating the query in question. However, not every author has a website (my techgirl heart shudders at the thought) and not every author puts agent info up. So sometimes there'd be a query or a partial or a full that we'd be interested about and my boss would email the author and we'd get a lovely response back saying, 'Sorry! I'm with someone else now.' It's like seeing a really awesome movie trailer that you get pumped up about and then having the studio yank the film. I took the time to watch the trailer, I got excited for the trailer, and now you're telling me no? It crushes my fragile soul.
This is a very long way of saying that if it were up to me, I'd say, yes, send every agent you queried an email telling them that you're no longer available. It makes life easier for the interns.
However, one must be practical about these things.
How long has it been since you sent in the query? Some agents have language on their website that say if you don't receive a response within a certain time period, you can assume they've passed. So if almost a year has gone by and you haven't heard from the agent at all, you're probably in the clear, but if you just sent the query last week, the polite thing to do would be to email and withdraw.
Was it unsolicited or did you meet them at a conference? If you've met them in person, it wouldn't hurt to let them know you've been snatched up by someone else. They may remember you and now they know you're a marketable commodity. You may choose to switch agents one day and you don't want to burn any bridges unintentionally. My boss remembers people. I'm crap with names so I might send her an email referring to the 'query with the duck and the mongoose' and she will be like, 'Ah, yes, Author X. I think they pitched me a book at a conference that I passed on because ducks weren't selling at the time and I told them to query me again if they completed manuscript-altering revisions.' So what do you have to lose by letting them know you are off the market?
Have you sent one hundred and fifty queries out? Kudos to you for being so dedicated to your craft. The previous questions still apply, but, goodness, would it really be time-effective for you to email the one hundred and twenty-nine agents who haven't yet replied to you? In this case, it would probably be better for you to focus your energy on polishing your manuscript to its very best state and giving the agents the bad news as they respond to your query.
Are you still hoping your dream agent might swoop in to sign you? So you got an offer of representation from an agent, but while you are very happy that your publishing dreams are moving forward, you still have a non-stalker-y author-crush on, say, my boss. If you have not yet said 'Yes, omigod, please represent me!' to the other agent, you can totally email my boss and be all, 'Just so you know, this other person asked me to the prom, and I haven't said yes yet, but if you don't answer me by the end of fifth period, I'm for sure gonna go with them.' My boss has actually gotten emails like these and those queries/partials/fulls moved to the top of the list. Nobody wants to go to the prom with a less than perfect date simply because they didn't ask in time. If you don't hear back from the agent within the time frame you specified, you are free and clear to make the decision that's best for you.
But don't actually ask my boss to the prom. I will find it hilarious. She will not.
My Boss Says: Yes, send an email (according to her, this would be the only acceptable time for a mass mailing). Put OFFER OF REP in the subject heading and be clear if this is a courtesy heads-up (i.e. you wouldn't be open to any other offers- think of it like an engagement announcement) or if you are trying to suss out other options.