According to THIS ARTICLE on Screen Rant, advanced copies of the Rise of Skywalker novelization were sold at his weekend's C2E2 in Chicago. They write:
Although the novelization isn't officially on sale until March 17, Lucasfilm Publishing decided to sell advance copies at this weekend's C2E2 in Chicago and passages have begun appearing online.
One of those passages explains the nature of Palpatine's resurrection. From the novelization:
All the vials were empty of liquid save one, which was nearly depleted. Kylo peered closer. He'd seen this apparatus before, too, when he'd studied the Clone Wars as a boy. The liquid flowing into the living nightmare before him was fighting a losing battle to sustain the Emperor's putrid flesh.
"What could you give me?" Kylo asked. Emperor Palpatine lived, after a fashion, and Kylo could feel in his very bones that this clone body sheltered the Emperor's actual spirit. It was an imperfect vessel, though, unable to contain his immense power. It couldn't last much longer.
There you go. Clone Palpatine. The old Expanded Universe is new again.