In defence, they generally have a euro price sticker on every book. There can be human error and some get missed.
However, even with extra costs, there should never be such a difference between stg & EUR prices especially on books. There's no vat difference and no customs duty.
My own belief is "paddy tax" was the primary reason Debenhams lost so much in Ireland as they used a €1.6 exchange rate. And people do notice and stores do lose business. Sometimes it's too late when they work it out.
For the consumer, the solution is to shop around and never be afraid to decline politely at the till and take your business elsewhere.
Btw, Dubray is owned by Eason.