When I finished my manuscript I wasn't sure of the steps needed in getting my baby to someone that would give it wings so it could fly out to all those readers just waiting to read it. If I'm honest I naively thought I could send out my manuscript in full and have someone just fall in love with it. The truth is there just isn't time, it is much easier for an agent(or assistant) to read through a shorter version, think Hollywood pitch, of your story idea.
So when asking how important a query letter is, the answer is simply, very. I struggled with the idea of taking something I worked on for almost a year and condensing it down to a page or two of coherent, intriguing text. I in fact started to hate the idea of a query letter, but the truth is it can be done. I have sent out versions of my letter and was nice enough to get a few critiques along the way. These, while disheartening at first, have helped me refine and spice up what I'm trying to say to the agent or publisher I'm sending it to. It's the foot in the door every new author needs to become published and as such I wish anyone at this stage in their writing as much luck as I hope wished back to me.
Here are a few links to sites I've found helpful toward building the perfect query.