What are the best introductory texts on modular forms that are suited for a brief six week course intended for advanced undergraduates? The students will be quite sharp and as far as prerequisites go, they can be expected to have completed two courses in algebra and analysis each as well as a first course in complex analysis and a course in number theory at the level of Niven-Zuckerman-Montgomery.

I am considering Apostol's Modular Forms and Dirichlet Series in Number Theory but worry that it may not be possible to make serious progress in six weeks.