[tedmiston]

> (a) How could you reliably track this if the user always keeps their Kindle on airplane mode?

Spotify's solution is to require a device to sync at least once every 30 days or the offline content expires.

> (b) How could you track this accurately if the user reads a few hundred pages in the subway, where there's no Internet service?

With a log of events that syncs when they do reconnect.

> (c) How could you distinguish that between someone who hopped around a book in that same time period?

By measuring the duration spent on each page.