"It's absolutely amazing," Esther, who runs the Dublin Bookshop in Ireland, told Newsround's website. "It's still all about Harry. If anyone comes in to buy something other than Order of the Phoenix, we'll be very shocked."
Mark, who runs the Borders shop in Speke, Liverpool, said the atmosphere was different to Saturday - but it was still great!
"Friday night was frenzied, but Saturday is all about a sense of fun," he said. "We've got loads going on - fancy dress competitions, live owls and snakes in the shop. We've never known anything like it before."
'Buzzing with excitement'
The Order of the Phoenix is expected to be the fastest-selling book ever. A spokesman for WH Smiths has already said it's broken all records and was "flying off the shelves at a rate of eight per second".
Graham, the manager at Printworks in Manchester, added: "It's still manic the next day! It's a bit calmer but everyone's buzzing with excitement.
"The main difference is on Friday lots of people were dressed up - Saturday people are 'normal customers'".