How do I read and write an iso-8859-1 file in Emacs 23?

[Alan Mackenzie]

Hi, everybody, 

the subject just about says everything.  Emacs 23 insists on fouling up
my text, converting (for example) ü ("u umlaut") into \374 each time I
try to save it.  It then complains it can't save \374 because it can't
"convert" it.

In desperation, I tried putting this on the first line of the text:

    -*- mode : Text ; buffer-file-coding-system : iso-8859-1-unix -*-

.  Should this help?  Is it causing me problems?

I've tried reading the fine manual.  It helps me not in the slightest.
What am I missing here?  All I want to do is read an 8859-1 text file,
edit it, and write it back again.  How do I tell Emacs that an 0xFC
character in the file is actually a "u umlaut", and not anything else.
Why is Emacs insisting on trying to be so clever?

-- 
Alan Mackenzie (Nuremberg, Germany).