I understand that we use हाथों because it is the oblique case. However, I don't understand how we know that the book is in both hands, not just one hand. Can someone write here how you would say, "The book is in my hand", so that I can see what the difference is? Thank you!
Found my answer - the oblique case for a masculine singular nouns not ending in "ा " does not involve changing the ending. So "The book is in my hand" would be किताब मेरे हाथ में है.
In English "is in my hands" is an idiom for "under my control"? Is it similar in Hindi? Or does this sentence only mean "I'm physically holding the book in my hands at the present time"?
This sentence only refers to holding the book physically.
However there is a related phrase in Hindi - मुट्ठी में होना - "being in a fist" that refers to something being in total control.
I think the English idiom you are thinking of is "in hand" which means to have control of a situation, e.g. "I have this in hand" or "I have the children in hand now"
That's a stronger idiom. "The situation is in his hands now" doesn't imply he has the situation "in hand" :)
Probably summarizing the idea of "in my hands" as "under my control" isn't adequate in general, but books are docile things that would have trouble getting out of hand on their own ;)
In that case you are not reading properly. My books have completely taken over my house.