I’m nearly at the end of the book I’m currently reading (ビブリア古書堂の事件手帖) and there are some phrases that have come up a few times.
1) 我に返る（われ に かえる）
to come back to reality (from your own thoughts/a trance)
2) 胸を撫で下ろす（むね を なでおろす）
On its own 撫で下ろす means to literally stroke something in a downward direction. However, the phrase 胸を撫で下ろす means to be greatly relieved.
3) 間髪を入れず（かんぱつ を いれず）
in no time, quick as a flash
4) 目を瞠る（め を みはる）
to open your eyes wide