> emacs -Q
 >
 > Two windows. Run `describe-function' on a function: The help buffer pops
 > up in the other window. `other-window' and click the link to display the
 > source file: my original buffer is gone. Very annoying.
 >
 > What I would like: Either a function that displays the source for the
 > function at point in the other window or for the *Help* buffer to open
 > the source file in the same window (i.e. the window which displays the
 > *Help* buffer).

I have written some code to do the former which you can find in the
attachement.  I haven't looked into it for some time but I suppose that
the function you want is called `fsap-find-other' there.

The latter should become easy as soon as the label argument for the
corresponding buffer display call is in place.  You then should be able
to choose any window (including the *Help* itself) for displaying the
source buffer.

martin