No one in their right mind looks forward to doing this. But, unless you're going to self publish, (and we'll get to that soon) you've got to do it.
You're whole book. In one paragraph. One paragraph small enough to fit onto a one page letter with at least three other paragraphs, two address blocks, a formal salutation, and closing. And no, you can't fudge the margins or use an eight point font. Call it less than one hundred words, possibly seventy-five. Eeep!
Now, I mentioned earlier that this paragraph is not the back of the book copy. But why not? After all, you are trying to sell your book to the agent, and the back of the book copy is trying to sell the book to everyone who picks it up.
And that's precisely why not. You are selling your book to one person. One person you can research. You can find that person's likes and dislikes. You know what books he's purchased before. You know what kinds of people he's worked with before.
In the case of my own book, and yours probably, there are a lot of angles you can play up when it comes to convincing your agent-to-be that you've got the book of his dreams. I can work on the fact that my main character is Jewish, the intense religious themes, and questions about theodicy and forgiveness if I aim at an agent who handles religious fiction. I can paint a story of an Epic Fantasy trilogy that will span modern day Urban Fantasy to High Quest Fantasy while weaving a classic love story between worlds, if I was hunting a fantasy agent. If I want I can minimize the fantasy aspect and show how it's dripping in erotic romance if I'm hunting an erotica agent.
So, as you sit there with your list of potential agents, take the time to write down what precisely they've sold, what it is they want in a book, and start to work on why your book is exactly what they want. At the same time (because, after all this is never easy) you need to keep in mind what your book actually is. In my case I may tone down the fantasy aspect, but if I leave it out completely and actually do get a manuscript request from an agent who handles erotic or religious fiction, I'll get a "no thanks" letter awfully fast, and be out the cost of a manuscript.
I'll once again borrow from the car shopping world. If you've ever had someone try to sell you a car that wasn't quite right, say trying to get you into one that cost too much, or had a bunch of things you didn't need, or was in much worse shape than claimed, you know how frustrating and annoying that is. You need to realistically assess you story. You need to know if you've got a Rolls Royce, a Subaru, a Honda or a Dodge and sell it as the best possible version of whatever it is you've got. Spending your hundred words talking about the aching beauty and immense clarity of human experience in your Literary Fiction story is great, assuming all of that is true.
So, how best to go about figuring out what your book is about? I suggest having a few people you know and trust read it and tell you. Seriously, what you think your book is about and what everyone else thinks it is about may be two very different things. Better yet, people you know and trust who you haven't already told what the book is about. (Because we all have talented reader buddies who we haven't talked the ears off of already...Yeah, I know exactly how realistic that proposition is. But assuming you have some people like that in your life, now is the time to ply them with gifts and a copy of your manuscript.)
Once you know it's time to write. And re-write. And re-re-write. And probably do it a few more times. Because here's the crux of this issue: this is the paragraph that makes your chance of ever getting an agent. There are a million ways of losing potential agents, and this is the one way to actually snag one. If you know people who have agents, beg them to read your letter, ask for brutal feedback. If you don't know anyone along those lines try to get people who will give you an honest answer to read your letter and answer this question: "Having read this, do you want to see my book?"
Lastly, good luck!