The Senate is still discussing whether it will hear witness testimony from John Bolton, the former national security adviser, who has said he would comply with a subpoena during the impeachment trial. But he’s planning to reveal some of what he saw regarding the Ukraine matter in his upcoming book, according to people familiar with the plan.

Mr. Bolton’s book, due to be published by Simon and Schuster, is almost finished, according to people familiar with his plans, and is set to be on sale well ahead of the Republican and Democratic National Conventions this summer. The book is going to describe Mr. Bolton’s time in the Trump White House and expand on at least some of what he saw regarding Mr. Trump’s efforts to pressure Ukrainian officials into announcing an investigation into Hunter Biden, the son of former Vice President Joseph R. Biden.