These are books that I have used and recommend for SysAdmins, even if you aren’t planning on sitting the exams.
CompTIA Network+ Study Guide
The CompTIA Network+ course gives a really good understanding of network principles and architectures. I used this book as a guide to help me through the content and hone my skills, repetitively using the practise questions until I couldn’t get it wrong!
CompTIA Security+ Study Guide
CompTIA Security+ provides a full overview of network security, and helps admins gain a foothold on the security ladder, learning how to design a network that is equipped to handle the vast majority of security threats.