I encounter a problem when I read compact complex surfaces.Let $X$ be a projective surface,$E$ is a vector bundle of rank 2 over $X$, $S \subset \mathbb{P}(E)$ is an irreducible subvariety such that $\pi _S :S\rightarrow X$ is birational. It say that $\mathcal{O}_{\mathbb{P}(E)}(1) \otimes \mathcal{O}_{\mathbb{P}(E)}(S)$ is trivial on all fibres of $\mathbb{P}(E)$, why?
Since $\pi_S$ is birational, it follows that $S$ is a section of $\pi$. Hence we can write $$\mathcal{O}_{\mathbb{P}(E)}(S) \cong \mathcal{O}_{\mathbb{P}(E)}(1) \otimes \pi^* \mathcal{L},$$ where $\mathcal{L}$ is a line bundle on $X$. This implies $$\mathcal{O}_{\mathbb{P}(E)}(1) \otimes \mathcal{O}_{\mathbb{P}(E)}(S) \cong \pi^* \mathcal{L}^{1}.$$ Since $\pi^*{\mathcal{L}}^{1}$ comes from the base $X$ of the projective bundle $\mathbb{P}(E) \to X$, it is trivial when restricted to any fibre. 

