It has three fonts I think, a serif similar to Times Roman, a sans serif similar to Arial and a courier analog.
igorsk is being modest, he's figured out how to add other fonts to the Reader's set, but as of yet it's a rather messy process (the thread is called "Reader Internals," I think, and it's a long and fairly technical one
). I'm one of the ones that is hoping that a self contained executable to handle managing fonts might emerge sometime soon.
