There are some unspoken rules of querying--or spoken but not spoken often/widely--that you should know before you start. Here are some key DOs and DON'Ts of querying.
DO carefully research & selectively query agents. Remember, don't query someone whose offer of representation you wouldn't want to accept. Always consider the best case scenario--an offer!
DO make sure your query fits on a single page. Your query should be about 250 words.
DO follow submission guidelines. An agent's website is usually the best place to find the most up-to-date submission guidelines. Cross-check there against whatever QueryTracker says.
DO simultaneously submit. Some agents will ask for or mention on their site an "exclusive submission," which means that you send your query/manuscript only to that agent for a set period of time, usually a few weeks or months. Agent exclusives are not in your best interest! You don't want one agent to hold your query/manuscript hostage for months or more when other agents could be taking a look. Query multiple agents and query widely (but mind the "query in batches" advice in the previous chapter.)
DO follow agents on Twitter but DO NOT pitch them there (outside of a pitch event). Also don't follow-up with agents on Twitter about your query/manuscript, or DM them (even if they follow you back).
DO tell an agent you are a fan of their clients' books, but only if you mean it! Don't say you love a book that hasn't come out or just sold. Be genuine, real, and honest, if you decide to personalize (you don't have to).
DO be sure to spell the agent's name correctly, and apply the correct gender. DON'T address your query to "Dear Agent" OR mass query! Queries CC'ed or BCC'ed to a bunch of agents will be automatically deleted by most of them!
DON'T attach your whole manuscript unless explicitly asked. When you query, even if an agent asks for pages, 99% of the time you are to copy and paste your pages directly into the body of the email. Follow the directions.
DON'T name drop someone unless they explicitly give you permission. This means that if you are friends with an author, you shouldn't mention them in your query unless you've talked to them.
DON'T insult YA, the agent, their authors, or any of the genres they represent. Hoo boy, this is a big one! Though I would hope anyone reading this wouldn't be inclined to do this. You don't make yourself look good by saying things like "Unlike most YA novels, mine has depth and complexity," or "This is much better than terrible YA books like Twilight." Apply to whatever your genre is.
DON'T compare your book to juggernauts like Harry Potter, or say your book will be a bestseller/make the agent a ton of money. Don't tell the agent what your book is/will be--that is the agent's job. Comparing your book to massive bestsellers and classics doesn't tell them anything except that you're likely not well read in your genre. Remember, comps should represent where your book will sit on the shelf and show the agent that you are well & widely read.
DON'T post/tweet/blog/vent online about the querying process/rejections/specific agents. A vague "ARGH QUERYING!" is okay but "so and so agent just rejected me and said XYZ I hate them" is not. Generally being super negative online about querying/agents/the industry is not a good idea.
DO follow up with agents if a) they respond to all queries but don't to yours (after a long enough period of time) b) they requested material and at least three months have gone by or c) they tweet/post that they are caught up on queries/material but you didn't hear from them.
DON'T follow up too often. This can be a slow process, unfortunately, so read the next section on nudges and follow the suggested timelines.
DO tell all agents with your book/query if you receive an offer of representation. This is a KEY piece of etiquette! If you receive an offer from an agent, you should immediately notify ALL other agents about the offer. This includes any agents with requested material, as well as those you queried but have not yet heard back from. One tiny exception to this: if you queried a "no response means no" agent and either that agent's deadline or a long amount of time has passed, you should not nudge them.
DO tell the first offering agent that you will need 1-2 weeks to make your final decision. This gives all the other agents reading an opportunity to respond. One week is standard, and depending on the timing of the offer/situation, you can go up to two weeks (in the case of a holiday weekend falling within the window, for instance).
DON'T accept the first offer you receive and not notify other agents. Generally, it's not advisable to accept the first offer you get, even if it's from your dream agent, without giving other agents an opportunity to read & counter-offer. Now, sometimes you just KNOW that you already have the offer you want, and you can indeed withdraw your manuscript from everyone else. But you may create frustration and hurt feelings. At the least notify everyone promptly.

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#HowToAuthor: Agents & Submission
Non-FictionAdvice for writing book-shaped things and getting them traditionally published. This series will cover everything from querying to agent fit, to building a platform and marketing yourself.