I guess this is what you need: FAT File System API | Low-leve FAT file system access for Windows Mobile devices by Wizcode

I'm not sure, but I think it's not free. Why do you need it? I guess you need something more than just file reading... Are you trying to make s browser or something? I think Windows can give you something that would simplify the job a lot.

BTW, when you don't know what to do - ask Google and read Wikipedia. (links)