The overall equation is pretty simple: If you want to understand network traffic, you really should install Wireshark. And, if you really want to use Wireshark effectively, you should consider this book. Already in its third edition, Practical Packet Analysis both explains how Wireshark works and provides expert guidance on how you can use the tool to solve real-world network problems.
Yes, there are other packet analyzers, but Wireshark is one of the best, works on Windows, Mac, and Linux, and is free and open source. And, yes, there are other books, but this one focuses both on understanding the tool and using it to address the kind of problems that you're likely to encounter.