The app-sci/maxima package installs Maxima's info manual into /usr/share/info/ and an Emacs mode into /usr/share/maxima/5.9.0/emacs/maxima.el. However, the latter refers to /usr/local/info/ for Maxima's info manual. Hence the Emacs mode does not completely work out of the box: the user has to set maxima-info-dir to "/usr/share/info/" in his/her ~/.emacs file.
Created attachment 36707 [details, diff] Changes info manual path in Maxima's Emacs mode. I don't know if this is the preferred fix, but it works for me.
Thanks for the tip. This is fixed in "maxima-5.9.0-r3".