This statistics page states that worldwide eBook in 2017 totaled $13B. It also states that US sales were ~ $6B.
On the other hand, this report from Author Earnings cites US sales in 2017 as being $3B.
This is nearly a 100% margin of error. It would be very much appreciated if somebody could help rectify this error. Which, if either of them, is accurate? How can I obtain more accurate eBook market value statistics?