Every electronics hobbyist is using a power supply of some kind. You might start with a simple battery, but sooner or later you get yourself a 'real' power supply. Probably (right behind after blinking an LED) it the most frequent DIY-project. I'm no exception for that - actually I did three of them. But they are all based on the venerable LM317, since its so easy to use.
The other part one comes into contact with all the power supply issues is when you need to power your project. One might get away with just 5V from a USB wall wart, but sometimes there are special needs (e.g. higher or lower voltages). In my projects I got quite fond of Texas Instruments Simple Switcher modules, and their power modules. For example, for my K8200 3D printer I used the PTN78020 6A switcher module to generate the 15V needed for the main board and the stepper motors:
The Raspberry PI controlling it is powered by a LMZ14203 Simple Switcher module. I even designed my own small PCB to hold the module (Disclaimer: this was part of my project for the 'Raspberry Pi DIN Enclosure road test:
Using these reduces the hassle of building your own switching regulator, and they are more reliable than the cheap modules you get at Ebay. You can use these modules more or less like a linear regulator (just need to be careful with the required capacitance at input and output), but get much better efficiency.
But as a hobbyist I still strife to learn more, and hopefully this book helps me at this. I learned quite a lot just from experimenting with TIs switch mode converter boards during my recent road tests, but with the book I can also get into all the theory behind them. The project I'm finally targetting is to build (yet another) power supply - this time a linear one with a switch mode pre-regulator.