After listening to your feedback, I've decided to release a free edition of Marvin for iPhone and iPod touch. The free edition has all the features of the full, paid version with the following two limitations:
- Your home and library screen will show one book.
- You cannot make in-app purchases (you wouldn't be able to "carry them over" if you upgrade to the full version later on).
You can get the free edition here.
Enjoy.