Ten copies of the book can be bought for a certain sum of money payable at the end of $2$ years. $12$ copies of the same book can be bought for the same sum in ready cash. What is the rate of interest per annum?

Let the price of one book in ready cash be Rs. $x$.
Thus, the price of $12$ books in ready cash is $12x$. This is the principal amount ($P$) available today.
The same sum can buy $10$ copies if paid at the end of $2$ years. This means the value of $10$ copies at the future rate equals the cash value of $12$ copies today, or conversely, the present value of the payment for $10$ books deferred by $2$ years is equivalent to the cost of $12$ books now.

Alternatively, viewed from the seller's perspective: the price of $10$ books with deferred payment for $2$ years accumulates an interest such that its total amount becomes equal to the value of $12$ books.
Let the cash cost of $10$ books be $10x$. The amount after $2$ years due to interest becomes equal to the cost of $12$ books, which is $12x$.

$$\text{Principal }(P)=10x$$

$$\text{Amount after 2 years }(A)=12x$$

$$\text{Simple Interest }(\text{SI})=A-P=12x-10x=2x$$

$$\text{Time }(T)=2\text{ years}$$

Using the formula $\text{SI}=\frac{P\times R\times T}{100}$:

$$2x=\frac{10x\times R\times 2}{100}$$

$$2=\frac{20R}{100}⟹2=\frac{R}{5}⟹R=10\%\text{ per annum}$$