Apple has published its 'iOS Human Interface Guidelines' as an iBook that can be downloaded from the iBookstore.
Apple tells developers that whether they're redesigning an existing app or creating a new one, they should consider approaching the job in the way that Apple approached the redesign of its built-in apps.
--- iOS embodies the following themes: ● Deference. The UI helps users understand and interact with the content, but never competes with it. ● Clarity. Text is legible at every size, icons are precise and lucid, adornments are subtle and appropriate. ● Depth. Visual layers and realistic motion impart vitality and heighten users' delight and understanding. ---