Hi Conor,
If your business belongs overseas and you send goods to the UK to individuals below the value of £135 then the responsibility to account for the VAT would be yours as the supplier.
If this is the case then there would normally be a requirement to register for VAT.
Please see the guidance below:
Changes to VAT treatment of overseas goods sold to customers from 1 January 2021
However the VAT liability of books in the UK is Zero and so you can apply for an exemption from registration.
Please see the guidance below, specifically section 2.4 and section 3.11
2.1 VAT — an introduction
https://www.gov.uk/guidance/zero-rating-books-and-printed-matter-for-vat-notice-70110#overview
If the value of the book is over £135 then it would be the responsibility of the customer to pay the VAT when the goods arrive in the UK and therefore there would be no requirement for you to register for VAT in the UK.
Thank you.