A school library bought 140 new books, resulting in $\frac{27}{25}$ of books. The number of books before the purchase, is:
(a) 1750
(b) 2500
(c) 2780
(d) 2140
(e) 1140
When buying 140 books, the library ended up with $\frac{27}{25}$ of the number of books, so 140 corresponds to $\frac{2}{25}$ of the books in the library. Therefore, we have:

$$
\begin{aligned}
& \frac{2}{25} \longrightarrow 140 \\
& \frac{1}{25} \longrightarrow 140 \div 2=70 \\
& \frac{25}{25} \longrightarrow 70 \times 25=1750
\end{aligned}
$$

The correct option is (a).