I don't know specifically what skills you're interested in, but this is all pretty much networking fundamentals. I think I learned most of what I'd need to do

I don't know specifically what skills you're interested in, but this is all pretty much networking fundamentals. I think I learned most of what I'd need to do this in 'TCP/IP Illustrated: Volume 1'. There's plenty of "network penetration testing" type books which might also be of interest, though I don't have a specific recommendation here. There's a wider set of books at https://github.com/jacobian/infosec-engineering https://github.com/jacobian/infosec-engineering that's pretty good too, though it's a wider set of things than this.