I'm sure most of the time when we talk about the books' chapters we are talking about a specific book.
I will check again but I am pretty sure we are supposed to read chapter two.
I wonder why "the" is not used before "chapter two".
In English, we normally never use definite or indefinite articles in front of things that are being referred to using numbers. That's, more or less, always true without exception. You can basically regard it as a rule.
I think the reason why we do this is probably due to the fact that numbers (only cardinal numbers, to be more precise) in general are used to identify things uniquely. And when something is unique (that is, there is only one instance of it which exists), you don't really need to use articles to help you determine which one specifically, out of many similar ones, you're talking about.
A couple of sentence examples:
Mr. Jones is in room 256.
You will find more information pertaining to this topic on page 234.