The code is written in the margins of an old travel book. It is a simple substitution cipher, where each letter is replaced with another letter. The key to the cipher is:
A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z
So, for example, the letter A would be replaced with N, B would be replaced with O, and so on.
If you know the key to the cipher, it is relatively easy to decrypt the code. However, without the key, it would be very difficult to crack the code.