The newly released Salesforce Touch Platform Guide provides a very useful consolidation of both technical information and design guidance pertinent to mobile development on the Force.com platform. Topics covered include Identity, Connected Apps, Mobile Components for Visualforce etc. All in all an essential read for anyone considering mobile app development.
Of particular interest is the content related to the Hybrid model of mobile application development (chapter 6 & 7), combining the benefits of native app development, i.e. on-device integration (Contacts, Calendar, local storage etc.), with the rapid development and ease of update of HTML5 development and web apps.