As the title suggests, feel free to mention books you feel every networking professional should own. For me I feel that two excellent books on networking are:
TCP/IP: Principles, Protocols and Architectures volume 1
by Douglas E. Comer
and
TCP/IP Illustrated, volume 1: The Protocols
by W. Richard Stevens
The first book is an excellent book to read from start to finish as it clearly describes networking basics and moves into more advanced topics as the book progresses. In fact if anyone is starting their CCNA studies I would recommend either reading this book first or at least reading through it as you progress through CCNA as it explains the concepts a lot better than Cisco IMHO.
The second book should be mandatory for anyone in the field as it is an amazing reference book. One of those books you will always want to keep nearby if you need to quickly lookup info on how a certain protocol works, what port it uses, etc.