Is it permissible to give them books containing Qur'anic Ayahs which prove the Oneness of Allah?
Given that these books are written in Arabic and translated into English.
Yes, it is permissible to give them books containing Ayahs of the Qur'an to prove the rulings, such as Tawhid (monotheism) and others, whether these books are in Arabic or translated. You are even praised (Part No. 2; Page No. 115) for doing so, because giving or lending them such books is considered a form of notification and Da'wah (calling to Islam). You will be rewarded if you do it sincerely.