I think someone else has explained the grammar to you. Suppose this is the past continuous tense, which means that you were reading in the library at some time in the past. You should understand that the following writing is probably normal for you.

I read in the library.

Do you think you will answer like this? Did anyone ask you what you were doing before? If you want to emphasize reading, you will say so, if someone asks you where were you before? What are you doing? How do you think you should answer normally? Isn't it:

I read in the library.

Moreover, reading is completely omitted in spoken English. There is no logic to say. Don't you go to the library to read?

I am in the library.

Have you ever thought about the way you omit your words?

Think for yourself:

Your mother asks you: Where were you yesterday afternoon? What are you doing?

You answered: I was in the library (yesterday)

[If you answer: I was reading in the library (yesterday), the following is gone]

[Don't you answer: I read a book yesterday! Still in the library! ! ! If you think this answer is normal, I surrender.

Your mother: What are you doing in the library?

You answer: reading, of course. Can I dance square dance in the library?

...