When installing Asterisk from source, you need to create an unprivileged user manually.
Add to /etc/group something like:
asterisk:x:110:
and to /etc/passwd:
asterisk:x:107:110:Asterisk PBX daemon,,,:/var/lib/asterisk:/bin/false
Just set unused GID and UID.
Then change permissions:
chown -R asterisk:asterisk /var/lib/asterisk/
chown -R asterisk:asterisk /var/spool/asterisk/
chown -R asterisk:asterisk /var/log/asterisk/
chown -R asterisk:asterisk /var/run/asterisk/
And finally, set the running user in asterisk.conf.
It is not mentioned in the Book, but I think chown‘ing /etc/asterisk/ with the -R option is also worth doing; changing config files (not directories, if exist) permissions in /etc/asterisk to 640 either.