He wrote that the value of the thing is defined by human thought. Someone thinks a thing has a value. The value is relative. However, all the values are generated by human's labor. Thus, the labor should be the unit of the value. I understand this book in
this way. An example is a value of linen. When a tailor made a shirt by linen, the textile weight or property doesn't change, however, the value (price) has been changed. The value of shirt is generated by labor.
Entropy indicates how the state is in order in physics/informatics. Let's think about cleaning a room. You can collect dust in a room, but the amount of dust doesn't change. However, the room state is in order instead of chaos/disorder. When you put your clothes in your shelf, this state is more in order than when your clothes are distributed on the floor. In this case, the amount of clothes doesn't change. In some sense, the state in order has more value than disorder state. A tailored cloth has more order than textile. In nature, the state is always one direction: order to disorder. Your room only goes to dirtier state and never goes cleaner state if you don't clean your room. (This explanation is ambiguous and not rigorous. If you want to know more about entropy, please look it up.)
When you tailored a cloth from a linen textile, or when you clean up your room, you decrease the entropy. I think Marx wrote that in many different ways. It seems his basic idea is that human labor is the only actor that can decrease the entropy. I would say it is true at that time. Therefore, he thought the labor is the unit of the value. I think some part of book should be written in mathematical formula, that makes easier to understand his idea for some people.
Assuming this hypothesis (= he was talking about decreasing entropy as a value), I can read the book easily. I thought why he didn't write easier first, but then I realized I learned the recent knowledge. When Marx wrote the book, the idea of entropy is not so common, also not really systematic. There was no easy way to learn at that time. At such time, he thought the value as entropy (or a similar idea) and he introduced such idea to economics. I can imagine that was a revolutionary idea.
This book was published in 1867. If the value is based on how much entropy can be decreased, we can have a simple question using a modern perspective, ``why the entropy is approximated by a labor time.'' This is too rough approximation. When the industry is based on a simple labor dominant work (as people work in a pipeline in a factory), this assumption might be OK. Everyone works in similar way, so measuring the value is based on the how long a person worked no matter what is produced by the work or who worked. Nowadays this assumption seems not right anymore. Marx might want to criticize the value evaluation by classes. Some noble people got more value even producing the same product. But a skilled person can produce more valuable products than non-skilled person. Creators like artists and authors are good examples. If I and a famous author worked on a writing exactly the same amount of time, the value of the produced books are completely different. If I and an experienced chef cook the same amount of time, the dishes by the chef should taste much better than mine. Moreover, we have now computers and robots. These machines can produce valuable things, which means not only human can decrease the entropy. If Marx saw the nowadays computer controlled factory, I am interested in what he would write.
I only read the half of the first volume, so I only saw the introduction of Capital. I haven't see why the subtitle is ``Critique of Political Economy'' yet. I would like to read further later. Still I felt I understood better with this idea, so I would like to introduce this idea to you.
One last interesting thing is about the German words Arbeitnehmer (employee) and Arbeitgeber (employer). According to his idea, only the labor can give the value, so the Arbeitgeber (the one gives the work) should be an employee. An employer only takes the value, so the Arbeitnehmer (the one takes the work) should be the employer. He wrote that he expected this mistake will be corrected soon. However, 150 years later, this mistake has not been corrected yet.