Basically, since regulated voltage would remain the same, you're really accounting for how many milliamps or Amps were consumed while the processor had uptime which could be expressed as Amp-hours, but more likely as millisecond or microsecond counts of varying power rates being consumed. If you multiplied that by voltage, you'd get watt/hrs or an equivalence for power, if you'd like.

Since regulated voltage should remain constant, it's probably sufficient to just account for uptime for each power state in a variable and multiply each of those by the respective amperage draw for that operation mode on a per uptime basis and subtract that from the total estimated capacity when first starting the device. Later as the battery was cycled you'd re-evaluate that total estimated capacity and replace it with measured capacity based on the method espoused above.

If all they're doing is putting an A/D converter on the raw battery voltage and estimating remaining charge from raw voltage under varying load, they'd better have a battery curve available to increase linearity if they want any kind of reasonable accuracy at all.

The methods where you estimate load based on what's active or actually measure current draw are considerably more accurate. OTOH, it's an ereader, not a life safety device...