WASHINGTON — Two years ago, the author Michael Wolff parlayed his access to one of President Trump’s most powerful advisers, Stephen K. Bannon, into “Fire and Fury,” a rollicking, behind-the-scenes account of Mr. Trump’s chaotic White House. The book sold more than four million copies, despite lingering questions about its accuracy.

Now, Mr. Wolff is back with a sequel, “Siege: Trump Under Fire,” which appears to rely just as heavily on Mr. Bannon. But the author’s source left the White House in August 2017 and has watched Mr. Trump’s circuslike presidency from afar since. That gives the disclosures in Mr. Wolff’s latest book a secondhand feeling — and one of his most sensational claims drew a quick, emphatic rebuttal.

A spokesman for Robert S. Mueller III, the special counsel in the Russia investigation, denied Mr. Wolff’s claim that in March 2018, Mr. Mueller was preparing to indict the president for obstruction of justice on three counts, including witness tampering. Andrew Weissmann, one of Mr. Mueller’s prosecutors, who Mr. Wolff says led that effort, did not even work on the part of the investigation that focused on obstruction.

In an author’s note, Mr. Wolff said his account of Mr. Mueller’s investigation was based on “internal documents given to me by sources close to the Office of the Special Counsel.” But in a rare on-the-record denial, the special counsel’s spokesman, Peter Carr, said on Tuesday that “the documents described do not exist.”