The Maine Center for Invasive Aquatic Plants has just put out an updated version of the "Maine Field Guide to Invasive Aquatic Plants." It features a key, profiles of the state-listed aquatic invaders, and even a section devoted to similar-looking native species. It is incredibly detailed and very nicely put together, and would likely be an asset to anyone interested in invasive aquatics. You can download the entire 90-page guide for free here (pdf), or if you want a full-color, water-resistant print version, you can order one for $13 + shipping. They even take Paypal!