My humble interpretation:
The eager man is keen to read and he will read the book well and enjoy it.
The tired man just wants the book, because he thinks it will be good for him, but , being tired, he will not be keen to read it or he will not read it well and /or enjoy it.
So, in the first part of the sentence, the emphasis is on "read" , and in the 2nd part, the emphasis is on book.
I suppose it is like someone who is keen to go to the gym and buy new trainers for that purpose, and the man who buy new trainers, without focusing on the purpose...
Maybe someone can come up with a better explanation