Ripple's validation of transactions / Ripple white paper? Is there a ripple white paper or some safe source to understand how the transactions are validated and made in the ripple network? 
VS Bitcoin, XRP is not mined, so how is the agreement reached? (I understand that there is some kind of prioritisation of transactions and the first one agreed gets validated to avoid double-spending, but how does that work if it is not via mining?) 
Is there some complete explanation to how Ripple works? Not the oversimplified stuff, but something that actually explains the whole process?


Thank you!
Is there a ripple white paper or some safe source to understand how the transactions are validated and made in the ripple network?


Yes. See consensus process and reaching consensus articles.


  VS Bitcoin, XRP is not mined, so how is the agreement reached? (I understand that there is some kind of prioritisation of transactions and the first one agreed gets validated to avoid double-spending, but how does that work if it is not via mining?)


Through consensus between validators (masternodes). They agree on which transaction goes first and sign the new ledger (block). This answer provides some additional details.


  Is there some complete explanation to how Ripple works? Not the oversimplified stuff, but something that actually explains the whole process?


The links above provides a good explanation.