***I'm not a teacher***
The book reads well means that the book is good to read and so, anyone can read and enjoy it. It points to the quality of the patient (the book) itself. The agent in this sort of constructions (middle voice) is unexpressed because it's 'anyone'.
For example, you can't say poems write easily. Writing refers more to the quality of the agent. Poems aren't easy to write for anybody who can write.
Middle voice seems to be something between active and passive voice. Let's consider a couple of examples:
1. These tiles lay easily.
Lay is a transitive verb that should have a subject and an object. Grammatically, tiles is the subject but where's the object? This is the feature of the middle voice - one of the participants is not visible.
Also, with transitive verbs you should have an agent and a patient. The agent does something and the patient undergoes the action. So tiles is clearly the patient. Notice it's not 'normal' for a patient to be the subject - that's another feature of the middle voice.
The patient is also the subject in passive voice but the grammar's different. In other words: middle voice has the same grammar as active voice but the subject is a patient like in the passive voice.
2. Woolen clothes wash easily.
3. Woolen clothes shrink easily.
(2) is an example of the middle voice, (3) is active voice. Why? Because in the middle voice, the verb has to be transitive and 'shrink' is intransitive. Woolen clothes shrink on their own, but they don't wash on their own. In the case of washing you need an agent; in the case of shrinking - you don't, which is why it's active voice.
In the middle voice you also need an adverb at the end. However, it cannot by any adverb:
4.*VW drives cautiously.
5.*VW drives dangerously.
(4) and (5) are incorrect because cars can't behave dangerously or cautiously. The adverb must describe the quality of the patient. That's why middle voice is often used as a marketing tool - if it refers to the quality of the patient, not the characteristics of the agent/user:
(6) VW drives easily.
The ease of driving comes from the quality of the car itself and has nothing to do with the driver or his skills - at least that's what they want you to think. ;)
Yet another thing - the subject in the middle voice has to be inanimate, so you didn't confuse it with the agent:
7.*Babies wash easily.
8.*My grandma frightens easily.
Technically, babies and grandma are patients here but it could be understood that the babies wash something or that the grandma frightens others, which is not the point.
9. Softwood saws easily.
10. Softwood is sawn easily.
11. People saw softwood easily.
In (9), the ease is due to the quality of the wood; in (10), the ease is due to the quality of the agent. (11) is rather awkward because it concerns people in general - everybody, and if so, why even mention that?
You could also say that (12) cadillacs sell themselves which suggests that selling these cars requires next to no effort, and again it's about the quality of the patient not the agent's skills.
P.S. I'm pretty sure the examples were taken from a book but unfortunately I couldn't find its title in my lecture notes.
On edit: I think I've found the source - Fellbaum, Ch. 1986. The middle in English. Bloomington: Indiana University Press.
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