I would recommend to choose one reference textbook to work your way through. Focus on the parts that you need to apply. There are also IEEE guides from the protective relaying committee that are practical and readable. Relay manufacturer documentation and application guides for the relays you would apply are also very helpful.
Besides reading, to really learn the craft of protective relaying, you should also have some technical mentors to talk design through with you and to review your work. Especially when you are starting out but in my opinion even as an experienced engineer, no consequential work (like relay settings) should be sent out without a peer review. Ideally this will be part of your organization's formal process. If not, take the initiative to seek it out.
If your organization has internal standards or templates to start from for relay settings, try to understand the reasons behind those standards or templates. Ideally these should be formally documented. If not, maybe you can be part of collecting the reasons for the settings from the people who came up with them and compiling it as written documentation, although this could be difficult to spearhead as a junior member of the team. At least you can ask some questions to try to understand the internal standards or templates. You have to understand the reasons behind a "standard" or "typical" setting in order to know when an exception to the standard might be called for.
The last thing I would mention for learning protective relaying is to review relay operations. After a trip or event, look at event reports from the relays to see what the fault looked like from their perspective. Find out what turned up in field investigation and compare it to what you inferred from looking at the relay records and what the textbook theory leads you to expect.
