According to my local paper's Twitter/X feed, HMRC has increased the 'side-hustle' reporting threshold to £3000. They explicitly mentioned people selling on market sites like Ebay and Vinted, but presumably it also applies to e-book sales via Bookapy.
ETA A further search revealed that side-hustle income over £1000 is still taxable by HMRC, which is introducing a simpler system for taxation of the profits rather than having to fill in a full self-assessment form.
AJ