I am looking for resources on fibres of continuous functions from $\mathbb R$ to $\mathbb R$. Here by fibres of a real valued function $f$, I mean the set $f^{-1}(y)$ for $y\in \mathbb R$. For context, I am trying to determine sufficient conditions on $f$ such that for every cardinal $\lambda$, the set $\{y\in \mathbb R:\,|f^{-1}(y)|=\lambda\}$ is borel.