Why you wrote the book may be compelling but it's not THE most compelling thing about the story (I hope!). It goes later in the query letter. Lead with your strongest element: the story.
Jessie MacGregor’s thirteenth year isn’t at all what she had in mind. She struggles with her mom’s recent departure from the family to “find herself.” Her dad hides his broken heart by burying himself in his work. He pressures Jessie to go on a teen backpacking trip, which turns into a rescue mission. When her hiking guide shatters his ankle, Jessie and two other teens must hike to safety from deep in the Sierra backcountry.
This is good.
Throughout her adventure, Jessie struggles with her mother’s search for herself and the angry terms on which she left her father. She wonders what the future will hold for her family and desperately wants to feel that she has some kind of control over her life. This inner struggle parallels her physical challenge of getting out alive and causes Jessie to develop an inner strength she never knew existed. Jessie realizes that whatever life throws at her, she will survive.
This isn't. You go from specifics to generalities. What happens on that trail? Is she attacked by bears? Struck by lightning? Solicited for an anthology of teen sex writing (ok, this is a bad joke but you get my point)
We need a sense of what she struggles with other than the obvious.
I have extensive wilderness experience, particularly in the Sierra Nevada, including numerous backpacking and mountain climbing trips, and a 270-mile trek from Yosemite National Park to Mt. Whitney. I’ve drawn on these experiences and research to create this high-action adventure novel.
This is actually better than the first paragraph about why you wrote the book.
I’m the proud author of seventeen children’s book...(publishing credentials to follow).
err...you have 17 books published and NOW you need an agent? Why?
My current goal is to obtain an agent to represent my novel, but I’m also interested in an agent who represents picture books and easy readers.
oh man. You don't need me to tell you that finding an agent who covers all these areas is gonna be tough. We have enough agents at FinePrint to field a baseball team, and even with that many I don't think we do picture books or easy readers. Sometimes if an author has a wide array of work, we can keep it all in house with several agents doing different projects, but this one is a tough problem. Yikes.
Thank you for your time. I look forward to hearing from you.