There two bugs in this chapter in the section "The defer statement".

The func Contents definition is this (line 9):
func Contents(filename string) (string, os.Error) {

and should be like this:
func Contents(filename string) (string, error) {

See the second parameters is not os.Error, but error.

The other bug is in line 22:
if err == os.EOF {

Should be now:
if err == io.EOF {

And you have to add the io import at the beginning.

Greets and nice job!