From its own cover,

It is written by experimental physicists and aimed to provide the interested amateur with a bridge from undergraduate physics to quantum field theory. The imagined reader is a gifted amateur possessing a curious and adaptable mind looking to be told an entertaining and intellectually stimulating story, but who will not feel patronized if a few mathematical niceties are spelled out in detail.

This might sound pretty casual, but it gets into all the math of it, with an aim at practical use.