This is a great question!
I've found the directions online to install it by hand unclear and, frankly, a bit of a pain (at least on Debian).
Emacs on Debian doesn't come with the Emacs Lisp Reference Manual by default. It's kind of silly, but Debian stores it in the non-free repos. Unfortunately, a regular
apt-get install doesn't Just Work™.
Note: Before proceeding, I assume you've installed Emacs from an official Debian mirror and not compiled from source or installed from some random
.deb from some random dude on the Internet. I also assume these instructions will generalize, but in case not, I'm working specifically with Debian 9 (Stretch).
To begin, you need to include non-free repos. Do this by appending
non-free1 to the end of each entry in
deb http://http.us.debian.org/debian stable main non-free
You'll need to run
sudo apt-get update to refresh the package listing.
Once you've updated the package listing, install the packages
X corresponds to your Emacs version, like
sudo apt-get install install-info emacs25-common-non-dfsg
I think that should do it, but for good measure, since I run
emacsclient, I restarted the Emacs server:
systemctl --user restart emacsd
Restarting Emacs might not be necessary.
I have not tested this, but it also ought to be possible to just point Emacs to the appropriate file. The source is called
elisp.info and is available at gnu.org. You'll want the "Info document" link. Check
M-x Info-default-directory-list to see where the default
Info directory is. You ought to be able to either place the
emacs.info file there or add another directory to the
(add-to-list 'Info-default-directory-list "/path/to/my/info")
You can read more about the info path on the EmacsWiki.
1 DISCLAIMER: Although the Emacs Lisp Reference Manual is libre, including non-free repos does open you to non-free software.
2 This should install the Emacs Lisp Reference Manual as
/usr/share/info. Strictly speaking, it seems like
dpkg is the only dependency, but I couldn't get Emacs to recognize the new contents in
/usr/share/info without install-info. Maybe someone could clarify why, but that's more a Debian question than an Emacs one...