The Impact of using Supplementary books alongside the national academic text book has received great attention of the curriculum and material developers. Since the beginning of language studies, Second & Foreign Language Acquisition (SLA & FLA) researchers have been searching for effective ways of improving learners' language skills. This study aimed at investigating the effects of using supplementary text books (Gaj and Khate Sefid) on English as a foreign language (EFL) learners' Reading skill. Data from this study demonstrate a significant role of using two kinds of books Gaj and Khate Sefid in improving the reading skill of participants in the experimental groups. The findings of this study may help the learners to enhance their independent English language learning and improve their reading comprehension and the overall learning of English by using Supplementary books.