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)