Boldface typing

[twlllmxxx]

Just learning emacs. Editing text, I want to type a word in boldface
and then go back to default. So I use M-o b and type in boldface, then
M-o d and continue typing. It looks right: one word in boldface and
the rest in default. Then I save the file. Then I re-open it, and the
boldface is gone. Maybe I made a mistake. I repeat the editing, save
the file, and then re-open it. Again the boldface is gone. Any ideas?

I'm working from Learning GNU Emacs by Cameron et al., O'Reilly.

This is GNU Emacs 24.2.1 (i386-mingw-nt5.1.2600).