This is the 27th most frequent Japanese word.
中
Middle; inside; during.
In this sentence, '中' denotes the interior of a location, specifically referring to being within the confines of the room.
彼は部屋の中にいます。
He is inside the room.
The usage of '中' here specifies the location where the activity (reading) took place, indicating the interior of the library as the setting.
図書館の中で本を読みました。
I read a book inside the library.
Here, '中' indicates the middle or duration of the lunch period, specifying a time frame within which an action occurred.