An oath breaker didn’t just harm an individual, they did harm to the whole community, and the whole community viewed them negatively.
What are those books?
A slightly older but enduringly popular work is In Search of the Indo-Europeans: Language, Archaeology, and Myth (Mallory 1989). I'm told How to Kill a Dragon: Aspects of Indo-European Poetics (Watkins 1995) is very good too, but I have not read it myself.
Chapter 2 of Indo-European Language and Culture: An Introduction (Fortson 2004) is a very good summary of PIE culture. The book is excellent in general - probably the best English language introduction to PIE linguistics at the moment - but outside the first two chapters it is not generally accessible to laymen.
If you read other European languages it's worth checking out other books that may be available to you. The field by definition requires the ability to read a decent number of languages, so the literature is spread out across English, German, French, Russian, etc.
Aaron Burr raised capital for a fake water company and applied for a banking charter for what is now JP Morgan Chase
some people are playing by a more effective set of rules and others are being lied to from a young age