A day after HarperCollins owner Rupert Murdoch came out in support of Macmillan in its stand against Amazon, a third Big Six publisher has sided with the embattled publisher. Last night, Hachette Books Group CEO David Young sent a letter to agents announcing he, too, would follow the pricing model advocated by John Sargent at Macmillan.

As Peter Kafka put it in a report at All Things Digital,

Here’s another publisher publicly throwing its weight behind Apple—and against Amazon—in the e-book pricing war. Hachette Book Group says it will pursue the “agency model” for pricing its e-books: They set the retail price, and the retailer gets a 30 percent cut.

Translated into more practical terms, it means that Hachette will demand that Amazon and other retailers—but really, this is aimed at Amazon—raise the prices on their e-books from the $9.99 standard they’ve adopted. Instead, it will want them to use the $12.99-$14.99 standard for new books that Apple introduced last week along with its iPad.

The only question now is: Are a bunch more buttons about to disappear from Amazon?