Literary agents will read the manuscript you send, and some the synopsis, but all will read the covering letter. Writing an effective one may take you a long time, but it is well worth the trouble.
The whole thing should:
Some agents and publishers acknowledge what they receive; others do not. Do bear in mind that some small agencies or publishers only deal with the unsolicited submission pile every few weeks, and so the waiting time may be slightly longer.
An agent’s advice
Here is the advice of literary agent Simon Trewin on writing an introductory letter:
"Life is short and less is more. No letter should be more than one side of A4 and in a good-sized (12pt) clear typeface.
Sell yourself. The covering letter is one of the most important pages you will ever write. I will be honest here and say I find selling myself very difficult, so I can see how tricky this is – there is a thin line between appearing interesting/switched-on/professional and arrogant/unreasonable.
letters that include phrases like “I am a genius and the world doesn't understand me” or “My Mum thinks this book is the best thing she has ever read”
(of course she does – that is her job!) don’t exactly fill my heart with
longing! In your pitch letter you are trying to achieve some simple things: you
want me to feel that you take your work seriously. Wear your writing history with pride. Tell me about
that short story you had published or that writing course you attended and the
fact that you are writing alongside a demanding job or in the evenings and
weekends when the kids are asleep. Tell me why you write – I love hearing about
paths that have led people to the moment when they think “I want to write”.
Tell me who your influences are and tell me about the book you are sending me. A few lines will do the job here; I just want to get a sense of the territory I am going to enter. Tell me what you want to write next. Hopefully you won’t be following your commercial romantic comedy with a three-volume science fantasy epic or vice-versa!
At the end of your letter I want to feel in good company and ready to turn the page. I am not interested in seeing what you look like or how old you are – we are not running a model agency here! Publishing isn’t as obsessed with age and beauty as you might think, but it is obsessed with finding distinctive new voices. And a final point: get a friend to read the letter and give you some honest feedback. Put it to one side for a day or two and come back to it – distance is a great editor."
Dear [Literary Agent]:
I am seeking representation for a non-fiction book entitled The Night Attila Died: Solving an Ancient Murder Mystery. I am a college professor with a PhD in medieval languages and literature from the University of Minnesota and a MFA in Creative Writing from the University of North Carolina. 
Historians tell us that Attila the Hun died on his wedding night in 453 AD. Drunk and flat on his back, he died of natural causes – an internal haemorrhage. The only problem with this account (and it’s a big one) is that it’s a complete fabrication. The Night Attila Died challenges 1,500 years of history by presenting evidence that Attila was murdered and that the truth was covered up in the official imperial records. 
The events and characters are among the most interesting that history has ever assembled on one stage. There’s Aetius, the ruthless Roman general and boyhood friend of Attila who defeated the Hun in a decisive battle in Gaul. There’s the weak and stupid emperor, Valentinian III, who pulled a dagger from his robe and assassinated Aetius in a jealous rage. There’s the emperor’s older sister, Honoria, who secretly plotted to wrest power from her brother and managed to start a world war in the process. 
In the eastern Empire, the characters are just as colourful: Emperor Theodosius II, a weak ruler who bungled the first assassination plot against Attila, and Emperor Marcian, whom I accuse of masterminding the plot that finally destroyed the Empire’s greatest enemy. Throw in, for good measure, a scheming eunuch and a pathetic little dwarf named Zerko. It’s a great set of characters. 
But what the book is really about is philology. The textual science pioneered two centuries ago by the Brothers Grimm is the tool that lets us peel away layers of conspiracy and propaganda. Through the philological method we can reconstruct what really happened and how the conspiracy to kill Attila was covered up as official history. Chapter by chapter the reader participates in the detective work. In the end the threads of an ancient conspiracy are revealed and the verdict of history is overturned. 
There’s more at stake than just a good detective story. This is ultimately about what happens when two cultures with irreconcilable worldviews collide. It’s how we confront the Other with all the power of the sword and pen. What emerges from these violent confrontations is a skewed understanding of the past. We may call it history, but it’s often just propaganda. The Night Attila Died is rooted in the historical moment of the late Roman Empire, but the conclusions I draw are deeply connected to our own time. 
My publications to date are academic, in particular a book on the literary representations of Attila. I am uniquely qualified to write The Night Attila Died, having spent 15 years studying the historical and literary records as preserved in Latin, Greek, Old Church Slavonic, Old Icelandic, Old French, and Middle High German. (But that isn’t keeping me from writing a lively narrative!) I am recognised as an expert in this field and have consulted for a History Channel documentary on “famous deaths”. As an enthusiastic and dynamic speaker who speaks widely at conferences, I intend to promote the book aggressively. 
May I send you a full proposal with a sample chapter? 
Michael A Babcock, PhD
Commentary (keyed to the paragraph numbers)
 Direct introduction. No beating around the bush. No ‘clever’ attempt to hook the agent. Identify the type of book it is. Briefly identify yourself and your credentials.
 The hook. What’s unique about this book? Why should the agent keep reading the query letter?
 What you’re trying to demonstrate in the body of the letter is your style, your personality, and the ‘interest factor’ of the subject itself.
With carefully selected details, you can pique the interest of the agent.
Agents and editors love books – that’s why
they do what they do. So show them what the pay-off will be for reading this book. You are also conveying the depth of the subject and your expert handle on the material.
 Establish the significance of the topic and its relevance. Establish points of contact with general knowledge (the Brothers Grimm).
 Again, this draws out the significance and timeliness of the subject – that is, you’re trying to answer the ‘So what?’ question.
 Return to your credentials and qualifications as to why you're the best person to be writing this book.
 End with a direct, unambiguous appeal that requests a specific follow-up action.
How it worked
‘This letter was sent out by e-mail to agents and out of the ten I submitted to, I heard back from nine and all nine wanted to see the full proposal. Of these nine I had three agents who were interested in representing the project and one, in particular, who pursued it aggressively. This agent called me up and expressed such enthusiasm for the concept and my writing style, that I felt he was the natural choice. Even though there were better known agents who were interested in the project, I opted for the lesser known agent on the theory that he was highly motivated to sell my book. The book sold in less than a month. There were three editors who were interested in making an offer on the book; in the end it came down to two and the higher bid won out. As a side note, the book sold on the strength of the formal proposal and a single sample chapter. The book was sold in December 2003 and submitted in final form to my editor in July 2004. It was published in July 2005 by Berkley Books.’
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