Yep, that is definitely a skill. And you are doing exactly what I recommend: slow down and take the time to read the thing and understand it. Then go forward from there. Your interest also shows that you are intrinsically motivated, which many people are not.

You’re right that there are very few books available showing people how to do this. Usually “Working Effectively with Legacy Code” is recommended but it’s just one book and so can’t cover everything. It’s an area where on the job training and experience definitely makes a difference.