Samantha is exploring new ways to motivate herself to read more. She considers subscribing to an e-reader service, which offers unlimited e-books for a $15 monthly fee, rather than purchasing individual e-books at $5 each. Previously, she read three books per month. Samantha believes that the subscription will encourage her to read more consistently. However, her friend argues that this plan won't be effective since the total monthly expenditure remains the same.

a. Evaluate whether the subscription service can be considered an effective commitment device for increasing Samantha's reading habits.
b. Discuss the implications by comparing the marginal cost and benefit of reading under both payment options