Having knowledge of Linux is essential for any system administration, middleware, web engineer job.
Linux is used almost everywhere in production or a non-production environment. There are thousands of article, book, video training to explore and learn but that would be time-consuming.
Instead, you can follow one or two related books or online training.
The following learning materials cover a large number of Linux Administration tasks from beginning to expert level. So pick the one suits you.
Source: 16 Linux Books and Videos for System Administrator