It's the building blocks of circuitry. As any redstone user may know, all "contraptions" are simply circuits, whether they are creating very basic stuff, such as Diodes (repeaters), Transistors (comparators), or more intermediate things such as Capacitors (Pulsers), orBoolean Logic Gates (redstone torches)...
The point is that using redstone is a system of logic.
You create "functions" that become tools in your tool box, then you get to use those tools to build contraptions to solve problems.
The best example is the question:
How do you make a hallway light switch?
The problem exists because you want a switch that turns-off and turns-on the light, from both ends.
This is not "hard", but, in circuitry it's also not as easy as you think it would be.
This book helps you solve those problems, expanding your tool box.
Then it's just a matter of translating it to your redstone creations.