This is the 2322nd most frequent Japanese word.
そしたら
"Then" or "if so."
In this context, そしたら is used to indicate a sequence of events, where the action of reading the book precedes the realization of its interest.
友達が勧めてくれた本を読んだ。そしたら、とても面白かった。
I read the book my friend recommended, and it was very interesting.
Here, そしたら introduces the result or unexpected discovery following the action of going to the station.
駅に行きました。そしたら、店が休みになっていました。
I went to the station, but the shop was closed.
In this example, そしたら shows the logical result or outcome due to the preceding event of having time to study.