The question is on the sentence as highlighted in this excerpt from L'Étranger by Camus.
J’ai dit qu’il [le chien] était de belle race et Salamano a eu l’air content. « Et encore, a-t-il ajouté, vous ne l’avez pas connu avant sa maladie. C’était le poil qu’il avait de plus beau. » Tous les soirs et tous les matins, depuis que le chien avait eu cette maladie de peau, Salamano le passait à la pommade.
How does the sentence mean what it means?
From English translations I understand that it means that the coat of hair was the best thing about the dog.
But how does it come to mean that?
Perhaps a way to rephrase the question is to ask how much freedom I have with the form of expression.
Can I substitute another adjective for beau? For example, could I say the following of someone with many charming qualities, but none more charming than her voice?
Elle a la voix de plus charmante.
Or can I even use any verb and any adjective? For example, if someone could gulf down any food, but hamburgers faster than anything else, could I say this?
Il mange les hamburgers de plus rapide.
Or perhaps I could even get rid of plus and say something like:
C’était le poil qu’il avait de beau.
Meaning that it was the coat that was a good point about the dog. (Suppose someone says the dog had good teeth, and you contradict it by saying the above.)
If I have all the freedom (the form of expression being flexible and not set), how could I say that it was in the coat that the dog outdid another? Supposing our dog was male, and the other female, could we say:
C’était le poil qu’il avait de plus beau que elle.
But that would mean de plus beau has to mean sometimes the superlative (as in Camus) and other times the comparative (as in our last example).
I realize that some of these questions may not make much sense. I am only trying to illustrate the sort of confusion caused in a beginning student by an unfamiliar form of expression.