This paper contributes novel measures of user engagement in mobile music retrieval, linking these to work in music psychology, and illustrating resulting design guidelines in a demonstrator system. The large music collections available to users today can be overwhelming in mobile settings, they offer 'too-much-choice' to users, who often resort to shufflebased playback. Work in music psychology has introduced the concept of music engagement -listeners vary in their desired control over their music listening, and engagement varies with listening context. We develop a series of metrics to capture music listening behaviour from users' interaction logs. In a survey of 94 music listeners, we show significant correlations between music engagement from questionnaires and the presented quantitative metrics. We show how music retrieval can adapt to this engagement, developing a tabletbased demonstrator system, with an exploratory evaluation.