Planes are my favorite handheld woodworking tool. Planes are essential to the woodworking process (they are used to create flat surfaces on the wood), and there are so many different styles, sizes, and adjusting mechanisms that it is very easy to quickly build a plane collection. You'll find the late nineteenth century littered with the patents of failed planemakers -- all of which makes their study that much more interesting. Before the planemakers started patenting things and making things with cast iron and steel, they were busy making all sorts of wooden planes which are also quite interesting to collect. Because of their popularity, there are several reference books published about planes.