After giving a discount of $12\%$ on the marked price of a book, a shopkeeper earns a profit of $21\%$. If he doesn't give this discount, then how much profit would he earn?

The relationship between Cost Price ($\text{C.P.}$) and Marked Price ($\text{M.P.}$) is:
$$\frac{\text{M.P.}}{\text{C.P.}}=\frac{100+\text{Profit}\%}{100-\text{Discount}\%}$$

Substituting the given values:

$$\frac{\text{M.P.}}{\text{C.P.}}=\frac{100+21}{100-12}=\frac{121}{88}=\frac{11}{8}$$

If no discount is given, the book is sold at the Marked Price, so new Selling Price ($\text{S.P.}$) = $\text{M.P.}$.
Let $\text{C.P.}=8$ units and new $\text{S.P.}=11$ units.

$$\text{Profit}=11-8=3\text{ units}$$

$$\text{Profit}\%=\frac{3}{8}\times 100=37.50\%$$