In the UK and Europe, adult coloring books are considered stationery.
A recent review by Her Majesty's Revenue and Customs in the United Kingdom has determined that UK sales tax (referred to as VAT) must now be charged on coloring books that are intended for adult use only.
Lightning Source is not set up to collect VAT for stationery and therefore has to remove all adult coloring book titles from distribution channels in the UK and Europe.
Further information on the current HMRC VAT rules for printed books can be found here.
These changes do not apply to sales tax rules in the US or those outside of the United Kingdom and the European Union.