A more complete and updated list may be found on the “Links”; section of the LPI web site.
The Linux Professional Institute: https://www.lpi.org/
Articles relating to certification:
https://www.linuxgazette.com/issue33/york.html
https://www.linuxgazette.com/issue34/york.html
https://www.linuxgazette.com/issue35/york.html
https://www.linuxgazette.com/issue37/york.html
Linux Training Resources: https://www.linuxtraining.org/
Linux Journal Forum: https://www.linuxjournal.com/forum
Other Linux certification programs:
https://www.linuxcertification.org/
https://www.digitalmetrics.com/
https://www.redhat.com/