What lies at the heart of your confusion is not really anything to do with past simple, or with grammar at all, but rather with the semantics of the verb
read.
You can make a distinction between two different senses of the verb
read:
telic and
atelic. When the verb is telic, you get the sense of completion that you're talking about. A test for whether the verb is telic is whether you can apply certain time frames to it. Imagine that a speaker wants to use the verb
read in the sentence
Alex read the book to mean 'all of the book' (i.e., in a telic sense). In this case, only the first of the two time frames in the sentence pair below makes sense:
Alex read the book in an hour.
Alex read the book for a hour.
In the atelic sense, the verb
read expresses only the action of casting eyes across text, and as such can have duration. In this sense, one can indeed read a book without finishing it, and only the second sentence below makes sense:
Alex read the book in an hour.
Alex read the book for a hour.