Absolutely. Keep writing. It's great to have finished that first book, but the hope is that with each story you write, you will improve. You'll learn new tricks, try new methods, think up better plots. And if that first story doesn't sell (though, of course, the eternal hope is that it will) then it is great to show you are dedicated to the craft by having another story drafted, polished and in reserve. And another. And, oh yes, this one over here. Yes, agents and editors like to know you aren't going to be a one-hit wonder (though this certainly doesn't mean you should pitch seven books to a single agent at once). Just take the best work you have now and put it out there. Then, during the weeks and potential months it takes for their response to come back, keep writing on that next work and keep honing your craft.