czytałem = I was reading
That will depend on the context.
Q: Co zrobiłes wczoraj?
A: Przeczytałem "Hamleta". ( we know the person read the whole play, stress is on the completion of an action)
Q: Co robiłes wczoraj?
A: Czytałem "Hamleta". ( we don't know if the the person read the whole play, stress is on the nature of action, not on result)
Q: Czy czytałes "Hamleta"? (stress is on the result, even though the verb is imperfective)
A: Pewnie, ze czytałem (I have read and I know its content since in this context the respondent will usually assume that the question is about whether he/she now knows the content of the play)
Example 3 has English equivalents. For instance:
Q: What books were you reading at school last month.
A: We were reading Hamlet, King Lear and a few others by Shakespeare.
Here, just like in Polish, there is no clear indication whether the books were read in their entirety, but the assumption will be that in fact they were.