Choosing an English course book which suits students in EFL/ESL settings is always a contentious issue for practitioners. This study is an investigation into the comparative critical evaluation of New Interchange Intro and New Headway Pre-intermediate series - two well-known series employed in EFL/EFL contexts - and some teaching challenges the teachers encounter during teaching these books in the context of Iran. The evaluation is done in terms of two assumptions; firstly, cultural and ideological assumptions, and secondly, assumptions about language, language learning and best practice. Findings reveal that both New Interchange and New Headway texts reflect ideological and cultural assumptions through their focus on the US and UK way of life respectively. The pictures and the material are found biased towards the culture of these countries in their depiction of local cities and lifestyles and in the inclusion of subjects. Regarding assumptions about language, language learning and best practice, the books focus on both form and meaning, and the grammar included is inductive and implicit. In the context of Iran, however, New Headway seems to be better for school students while New Interchange would be suitable for students and persons who aim to migrate or travel overseas. The study gives some suggestions for improving the usability of these books in the context of Iran.