Wikipedia of course has a good cursory introduction on the subject, but PLLs are so dynamic that it's hard to really grasp the concept regardless of how many articles you read on it. Horowitz and Hill of course do a decent job in The Art of Electronics, and the ARRL Hardbook adds some insights on the subject with regards to analog applications, but I still didn't quite get it. Jeri Ellsworth hosted a video on the subject, but even that didn't quite give me the intuitive understanding I was looking for.
4000 logic series includes the 4046 PLL chip, which includes both a voltage controlled oscillator and both types of phase detectors. After putting it all together, I figured I'd record a video and let you enjoy the play-by-play as I try and break it all down for you.
FM PLL Demodulation Video:
Of course, it's hard to understand why we're so excited about this phase-locked loop without a solid example, so as a follow-up to the first video, I made some alterations to use this VCO and PLL to use it to transmit frequency modulated audio.