Princess Diana's Brother Charles Spencer Debuts the Sequel to His History Bestseller (About a Royal Named Charles!)

The new book explores a stunning real-life tale

Charles Spencer is a part of royal history himself — and he’s also an expert on the kings and queens of the past.

The author and historian’s latest book, To Catch A King: Charles II’s Great Escape, debuts this week and is a follow-up to his 2015 bestseller Killers of the King: The Men Who Dared to Execute Charles I. The book was celebrated at an event in London on Thursday at the Ralph Lauren store on fashionable Bond Street. (The location was fitting: Charles II loved clothes!)

The 9th Earl Spencer noted that the monarch once bought 10 purple capes and purchased as many as 40 suits in one year. At one point, Spencer said, Charles II spent 1 percent of the nation’s wealth on himself in a single year. (Really.)

Charles II was on the throne during one of the most revolutionary periods in British history: To Catch a King tells the story of the search for the monarch, who was on the run after he was defeated by the armies of Oliver Cromwell.

The king’s flight lasted over six weeks throughout England. He famously stayed in an oak tree and in country homes of allies, which is captured in all its gripping adventure in the book.

To Catch a King is Spencer’s sixth book. His previous works cover the execution of Charles I, a biography of Prince Rupert and the story behind his family’s ancestral home, Althorp. Of course, as well as writing, he heads up the family estate, where his sister Princess Diana is laid to rest.

As he finished writing the book earlier this year, Spencer also marked the 20th anniversary of Diana’s death. He spoke to PEOPLE and ABC about her legacy for the documentary The Story of Diana.

Spencer was joined at the book launch by his wife, Karen Spencer, who is the founder and CEO of Whole Child International, his sister Lady Sarah McCorquodale, British Foreign Secretary Boris Johnson, actress Joan Collins and historian Kate Williams.