In particular, there is no real reason why certain changes happen while others don't. For instance, the * p sound from Proto-Indo-European became an * f sound in Proto-Germanic. Eventually, this explains why Germanic words like English father, fish, and foot correspond to Latin or Romance words like pater, pisc-, and ped-.
We know that in many languages, /p/ changes to /f/ because it's easier to pronounce.But we can't explain why /p/ changed to /f/ in Proto-Germanic and no other branch of Indo-European. There is no why.
1. The title of my question concerns the sentence coloured in grey above. I already listened to audio clips of these IPA symbols here.
2. Which subfield of linguistics comprises questions like this? What are some good introductory books, written for the layperson? I'm guessing historical linguistics and phonology?