“I am to read a book.”
Please specify if it’s correct and if no, then specify the reason.
Thanks in advance.
To me that means “I’ve been commanded to read a book” or “I am expected to read a book” – in other words, you’re reading the book because you’re supposed to read the book… as if it’s a requirement for completing a class.
If this decision is solely yours, I’d say “I’m going to read a book” or “I am about to commence with the reading of a book” (as Bill Cosby might say) or, very simply, “I’m reading a book”.
Even “I’ve decided to read a book” would be fine. In fact, if you’re not actually reading the book – but have made plans to do so – this might be the best option (or at least better than the others I’ve listed).
For your understanding of regional differences in usage (do not actually speak like this unless you are in this area of the United States):
You would say “I’m fixin’ to read a book.”
fixin’ is how “fixing” is commonly pronounced in the South (southern USA).
Although I am in eastern Japan, well, I don’t hate it! 
I say to my American friends ‘I’m kiddin’’ more often than ‘I am kidding’ or ‘I am joking’. 
Your information is informative, Tom.
Haihao
Yeah, I understand.
All I wanted to know is that if it were grammatically correct since my mother and brother, (both are English teachers themselves), argued with me that such a thing is grammatically incorrect and using it is inappropriate.
Of course, I understand that it’s usage in common English is not vast; however, if it’s grammatically correct, than it would appropriate of me using that phrase, for an instance, when given the question “What are you planning on doing right now?”.
A question that has been troubling me recently
Can I consider the phrase “I am to do something” as the short of “I am supposed to do something”.
Hi,
It also suggests that someone has told you to do something and there is also a suggestion that you don’t really want to do it.