As a writer, professional editor, and former intern for one of the top literary agencies in New York City, I've seen my fair share of query letters. A number of minor issues tend to pop up fairly consistently, but there's one mistake I see writers making again and again.

And this mistake could easily cost you a literary agent.

The biggest mistake that writers make is sending out a query letter that's way too long.

Before I delve deeper into the issue, I'd like to define what a query letter is and what exactly it should accomplish. A query is a snapshot of your project that should entice an agent into reading your manuscript. It's also an opportunity to show the agent who you are and assure him or her that you'll be professional and pleasant to work with.

That's it. That's all a query really needs to do. Yet I've seen so many writers include every detail of their plots, craft a meandering and overly comprehensive bio, and generally say way too much about themselves and their projects.

A fiction query should be between 250-300 words. You can usually get away with stretching this to 350 for nonfiction or memoir so you have room to include some information about market and audience.

Yet I continually see queries spanning anywhere from 400-600 words. When a query is this long, most agents won't read past your first few paragraphs, or they'll simply skip your letter altogether.

To agents, a long query appears amateurish and doesn't reflect the kind of professional and developed writing skills they're looking for in a client. Plus, agents receive hundreds, if not thousands of queries every week. They simply don't have time to read a long letter.

Here's a brief rundown of what your query should include:

A sentence of introduction, including your hook, genre, and word count.

A paragraph (or two at most) with a tight and engaging synopsis of your plot.

A few sentences for your bio with only the most pertinent and impressive information.

A line of personalization explaining why you're querying this particular agent.

If possible, it's also good to include some comparative titles and address who you think your book will appeal to.

Is it easy to accomplish all of this in such a short space? Absolutely not. Writing a query letter is a craft and a process. It can take hours, if not days, to compose a great query, and it shouldn't be done without outside feedback. But considering how long you spent writing your book, putting significant time and effort into the query letter is undoubtedly a worthwhile investment.

If you spend enough time drilling down to the heart of your story and what makes you unique as a writer, it is possible to distill your project into a coherent and compact letter that an agent will enjoy reading. If you achieve this delicate balancing of elements, there's a good chance that the agent will move on to your pages and eagerly request your manuscript.

Need help crafting your perfect, engaging, and concise query letter? Check out my upcoming "Query 101: How to Land an Agent" two-week crash course starting on February 6th, 2016. I'll cover everything you need to know about writing a killer query and provide a comprehensive and detailed critique of your letter.