Book reviewing is a commonplace activity on many ecommerce sites. However, it is nested within the broader context of book buying and selling. Goodreads, an online platform for social curation of book collections, provides an opportunity to observe on-line book reviewing in an environment that is not (at least overtly) focused on commercialization. In this study, we perform a careful comparative study of reviewer behavior and engagement in Goodreads and Amazon.com, constrained to a single genre (biography), including 21,394 books and 2.5 million reviews. We discover marked differences between the platforms that suggest disparate population composition and objectives of review-writing across the two platforms. Our findings suggest an important and generalizable principle: that two platforms engaging users on the same task (e.g., book review writing) may elicit quite different behavior depending on the implicit or explicit context and motivation present.