Being an XML file, you can use different editing tools to edit a style. The first choice should be your preferred text editor, for example, vi , emacs , or notepad++ . Consider that as you add rules and symbols, things may become fairly complicated. A tool that has highlight syntax for XML may greatly help you in debugging your styles. Of course, if you are trained to use it, a specialized XML editor that has support for XSD validation may help further, but usually I find it overkill.
Talking about editing styles, we shouldn't forget to mention the GeoServer administration interface. Indeed, GeoServer includes a simple GUI to view and edit XML files containing style rules. It contains a rich editor and a SLD validator; you got a first look at it in Chapter 3, Exploring the Administrative Interface.
Apart from XML/text editors, you can also consider a GUI tool to create styles; some open source Desktop GIS may produce SLD files. For example, QGIS may translate a layer legend...