[Solution] Let the prices of the books from left to right be $p, p+2, p+4, \cdots, p+60$. The price of the book in the middle is $p+30$.
According to the problem, we can set up the following two equations:

or
$$
\begin{array}{l}
(p+30)+(p+32)=p+60, \\
(p+30)+(p+28)=p+60 .
\end{array}
$$

The solution to the first equation is $p=-2$, which does not make sense in the context;
The solution to the second equation is $p=2$, which means the adjacent books are on the left side of the middle book.
Therefore, the answer is $(A)$.
$2 \cdot 431$ books are arranged from left to right in increasing order of price, with the price difference between adjacent books being 2 dollars. For the price of the rightmost book, a customer can buy one of the middle books and its adjacent one, then
(A) the adjacent book referred to above is on the left side of the middle book.
(B) the middle book is priced at 36 dollars.
(C) the cheapest book is priced at 4 dollars.
(D) the most expensive book is priced at 64 dollars.
(E) none of the above.
(12th American High School Mathematics Examination, 1961)