Actually the Prophet saws didn't tell him to learn jewish material he was told to learn hebrew, the language of the Jews so that the Prophet saws would have a translator that he could trust when he was corresponding with the Jewish tribes.
The Prophet then set him a task which required intelligence, skill and persistence.
"Zayd, learn the writing of the Jews for me," instructed the Prophet. "At your command, Messenger of Allah," replied Zayd who set about learning Hebrew with enthusiasm. He became quite proficient in the language and wrote it for the Prophet when he wanted to communicate with the Jews. Zayd also read and translated from Hebrew when the Jews wrote to the Prophet. The Prophet instructed him to learn Syriac also and this he did. Zayd thus came to perform the important function of an interpreter for the Prophet in his dealings with non-Arabic speaking peoples.
