While James Maxwell is recognized as the giant who showed the relationship between electricity and magnetism, Oliver Heaviside was largely responsible for consolidating Maxwell's theories into a manageable set of four equations and making them accessible to both scientists and engineers.
Heaviside started in telegraphy which was based on simple ideas of the flow of electricity without recognition of the effect of capacitance and inductance. Odd effects, such as variations in maximum keying speed and cable faults that actually improved throughput, were not understood and caused Heaviside to start investigations.
Maxwell's work was delayed in adoption as his proposed mechanism for energy transfer was the field, while current scientific thinking was that it was due to action at a distance. However, Heaviside was one of a smaller group that embraced Maxwell's ideas, allowing them to demonstrate new effects such as the creation of radio waves. This group, often called the Maxwellians, were Heinrich Hertz, Oliver Lodge, George Francis FitzGerald and Heaviside.
Heaviside's contribution was to cut through the complex math of Maxwell's work, introducing vector analysis of the fields and thereby making their application to engineering problems more accessible.
Heaviside was the first to introduce the idea that communication lines could be made distortionless by balancing the capacitance with inductive elements. Unfortunately, he presented this important concept as a minor part of his writings, and others reaped the gains of this advance.
Overall, an excellent book on the Victorian development of electromagnetic theory. Mahon has kept the equations to a minimum and included many metaphors to make the concepts easier to understand.