Some of the many books on the subject:
@appassionato @janneke
'The command line' is not a topic but just a specific way of interacting.
I guess it's the `bash` (bourne again shell) that is meant. The proper way of learning it is running linux (from a live image on a USB stick if you don't want to install it), to open a terminal ("the command line") and simply type `man bash` or `info bash`.
Also nearly all bigger distributions offer some kind of introduction on their webpage. Take the one of the live image/ distro that is used.