Four years later, as an informant for Britain’s Secret Intelligence Service, known as MI6, he passed on information about systematic graft in the Madrid headquarters of the G.R.U., as Soviet-era discipline relaxed. He told MI6 the identity of certain officers who were overcharging for procurements and inventing fictitious budget items, then distributing a cut to senior officers who served as their protectors.

Britain used Skripal’s information to help Spanish intelligence recruit the naval officer — the man who, in 2004, was found strangled in his hospital bed. Mr. Skripal’s MI6 handlers offered to remove him from Russia, the book claims, but Mr. Skripal decided to return to Russia, where, two months later, he was arrested.

During about 10 hours of interviews with Mr. Urban, in a house stocked with the jigsaw puzzles and model ships of an aging, solitary man, Mr. Skripal said he was afraid to attract attention by being quoted in the book.

“It’s because of Putin,” he told Mr. Urban. “You see, we are afraid of Putin.”

Mr. Urban, who has covered Britain’s security services for 30 years, frequently presents the thinking of intelligence officials without naming them. A portion of the book is told from the perspective of the MI6 officer who recruited Mr. Skripal, who is identified with a pseudonym.

Mr. Urban does not settle on one theory as to why Mr. Skripal, a relatively obscure figure, was targeted for such a spectacular attack. But one theory in the intelligence world is that Russian hit men were unable to locate their first choice — Col. Aleksandr Poteyev, deputy head at the Russian Foreign Intelligence Service’s American section, who in 2010 blew the cover of 10 Russian undercover spies living as Americans.

The C.I.A. spirited Mr. Poteyev out of Russia in July 2010 — the very week that then President Dmitri A. Medvedev of Russia had visited Washington for a smiley public meeting with President Barack Obama — and then broke the news to Moscow that 10 of its undercover spies had been betrayed by a Russian officer.

Mr. Putin, as a young man, had served in a role similar to Mr. Poteyev’s, providing support to undercover agents stationed abroad. He seethed with anger when asked about the betrayal, at one point saying at a national news conference, “How will he be able to look in the eyes of his children, the pig?”

Mr. Urban suggests that Russian intelligence services may have had trouble finding Mr. Poteyev, who had been resettled under an assumed identity by the F.B.I., and then “moved down the list” to other former agents deemed traitors who were easier to find — like Mr. Skripal, who lived openly in Salisbury.