One thing that I've learned from seven months of querying and submitting, is that less is more. You need to say enough to intrigue someone with the premise of your story, but not so much that you bore them. They will also be checking anything that you write in your letter, synopsis and chapter sample for spelling, formatting and style.
Concision is all. Most agents specify one or two pages at the most for the query letter. This caused me some aggravation recently, when I had to hand write an approach to Cornwall's only literary agent, a stipulation that she made. I was well out of practice in calligraphy, as the only penwork that I do these days are scribbled reminder notes and shopping lists. Using a fountain pen on posh handmade A5 paper, which I had only four sheets of, made me consider my words carefully - particularly as my contact details took up a third of one sheet. I managed it, despite ruining two sheets, and she got back to me requesting a writing sample.
We all want to be honest and to sell our book and ourselves, but that can be done briefly and neatly.