On Thu, Feb 14, 2019 at 08:10:54AM +0100, Marcin Borkowski wrote:
> Hi all,
> 
> I am writing an interactive command to be used while in the minibuffer.
> I need to know the buffer I was in when I issued the command that put me
> in the minibuffer (like M-: or M-!).  I tried (other-buffer
> (current-buffer)) and (last-buffer (current-buffer)), but to no avail.
> Then I re-read the docs for `other-buffer' and used this: (other-buffer
> (current-buffer) t), and it seemed to work.  The question is, how
> reliable it is.  Am I doing this correctly?

There's also minibuffer-selected-window (Emacs 27.0.50 here):

  minibuffer-selected-window is a built-in function in ‘C source code’.

  (minibuffer-selected-window)

  Return window selected just before minibuffer window was selected.
  Return nil if the selected window is not a minibuffer window.
  This function does not change global state, including the match data.

HTH
-- tomás