Question

1. Books are expensive. [Books in general]
2. The books are expensive. [The specific books, The particular book...]
3. The book is expensive.
4. The book in general is exprensive.
5. The specific/particular book is expensive.
6. I bought the book at the bookstore. [the specific/particular book]
7. He likes reading a book. [a book in general]
8. He likes reading books. [books in general]
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It seems that the explanation of #1 and #2 is correct. What about #3?
Does #3 mean #4 or #5? Does #7 mean #8? Is the explanation in [ ] okay?

Answer 1

#4 should read "A" book in general, is expensive.
#7 works when it refers to The person likes reading a book versus reading a magazine.

Answer 2

Yes, your explanations are good. You may want to add a wee bit to your explanation of #7 such as [a book in general versus a magazine or other print materials]

Answer 3

I agree with MsPi that #7 is just a tad off. Here are some other possibilities:

He likes reading a book before going to sleep.
He likes reading a book rather than a magazine.
He likes reading a book instead of watching television all evening.
He likes reading a book before seeing the movie that was made from it.