Speaking later to reporters after his release, Shadrake said he had hardly slept "since they dragged me out of bed" on Sunday, the morning after launching his book at a private function.
"I've had a few hours of sleep on a very hard floor and I've been sitting at the desk being interrogated all day long explaining all the chapters of the book and going through the history of the book, my research, why I did the book."
His passport has been confiscated to prevent him from leaving Singapore until the case is resolved.
The author, who divides his time between neighbouring Malaysia and Britain, is facing charges of criminal defamation and contempt of court, which are punishable by jail and fines.
|Singaporean authorities said the book had not been banned in the country [EPA]
The case is to be heard in court on July 30, his lawyer said.
Critics say Singapore has one of the world's highest rates of executions but the government refuses to disclose any numbers and maintains that capital punishment helps keep crime rates low.
Shadrake was arrested after the launch of his book "Once a Jolly Hangman: Singapore Justice in the Dock" in which he alleged double standards in the Southeast Asian nation's use of the death penalty.
Amnesty International earlier urged Singapore's government to immediately release the elderly author.
The book features interviews with local human rights activists, lawyers and former police officers on various cases involving capital punishment in the city-state, which carries out the death penalty by hanging.
It also contains an interview with Darshan Singh, the long-time chief executioner at Singapore's Changi prison, who has since retired.
The crime rate on the island nation of five million people, which imposes the death penalty for crimes such as murder and a mandatory death sentence for drug trafficking, is among the lowest in the world, and Singapore officials say the death penalty is a key factor in maintaining that.