1) I don't like this expensive a car.
Does it mean that I don't like all the cars of that price category?
2) I have been reading a book for the last two months.
May it also mean that I have been reading more than one book?
I also think that way. Why do you think 1 isn't natural?
From google books
1) I don't think that I made this big a fuss for my wedding.
2) "I can't believe he's making this big a deal over $3,000," she said.
3) The truth is, John had never eaten in this expensive a place before.