There are a lot of do's and don'ts when it comes to writing query letters. Sometimes you'll see the suggestion that you should compare your book to another one that is out there, or your writing to a popular writer. I know they suggest this for marketing purposes, but I've never done it. For one thing, what if the agent or editor to which you are querying doesn't like the book or the writer you are comparing your work to? What they really want to know is the audience you are writing for, so you might say, "I believe my book will appeal to readers who enjoy Mary Higgins Clark," etc.
In The 101 Habits of Highly Successful Novelists, Bill Pronzini is quoted as saying: "Always do your own work. Never try to imitate favorite or bestselling authors. Never follow current trends; what is a hot topic today may well be ice cold by the time a novel is written and submitted for publication. Imitators are seldom successful. An individual's unique style and vision are what editors are looking for."
So don't try to write the next Harry Potter series. You couldn't possibly improve on it anyway. Don't tell the agent that your friends call you "the new Louis L'Amour," unless you want to brighten their day with a lot of laughter. Just write the story that is in your heart and write it in your own unique voice. Will they like your story? There is no way of knowing. But I do know one thing: you can't put your soul into another writer's vision.