Search for the eBook in Library Discover then look out for instructions given in a Public Note in the View It part of the screen, e.g. To login off-campus, Select "Log in", then "Institutional Log-in", and finally "Athens Login page".
This will direct you to login with your LJMU username (in the format: username@ljmu.ac.uk) and password.
There may also be instructions under Authentication notes, e.g. Available off-campus with your LJMU username and password.