I'd recommend Kurose's Computer Networking: A Top Down Approach (now in 8th edition) book if you want to learn a proper computer networks concepts because this is the definitive textbook.

If you want the practical aspects of computer networks, you can try Niall Manfields' Practical TCP/IP designing, using and troubleshooting TCP/IP networks on Linux and Windows book. Granted it's a bit old (2nd edition is essentially the same 1st edition), but because you are after the essentials it should be more than fine. Here's the review of the book:

https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=122...