All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If a person is a knowledge seeker who is well-versed in Islam, he is permitted to read those books in order to find out the falsehood which they contain and then he would clarify to the people the truthfulness of Islam and that all other existing religions are false religions. He should also clarify the truth that some of those books may contain confirming the truthfulness of the religion of Islam, giving glad tiding of it and constituting evidence against the people of those religions.
As regards the normal Muslims, then they should not read or investigate those books as this may lead them to doubt their religion and cause them to have misconceptions about it if they read those books before being qualified and knowing the truth with its evidence and proofs.
For more benefit, please refer to Fatwa 83344.
Allaah Knows best.