These are all useful. If you're more of a book worm check out Minix as said in other parts of this tree. Very very easy to read, interesting bits of history about OS development, and source code included! Nothing's useful without source code.
If you'd like to look up some general software books check out Programming the IBM PC, or something to that effect. It's commonly called The Pink Shirt Book. Has interesting stuff on IBM interrupts, coding standards, and bits on filesystems!