The man arrested after apparently plotting to assassinate former President Donald J. Trump at his Florida golf course on Sunday was listed as the author of a rambling, self-aggrandizing, self-published 2023 book in which he appears to tell readers — and perhaps Iran — that they were “free to assassinate Trump.”

The book credited to the suspect, Ryan Wesley Routh — “Ukraine’s Unwinnable War” — aligns closely with Mr. Routh’s social media posts and public statements reflecting his desire to fight and possibly die for Ukraine, and his searing hatred for President Vladimir V. Putin of Russia.

Many of the book’s 291 pages contain graphic and bloody images of soldiers and civilians from conflicts around the world. The prose is written by someone who seems to fashion himself an expert on world events, and not as a peripatetic pro-Ukraine activist and building contractor with a criminal record.

Mr. Routh, 58, took a particularly dim view of Mr. Trump, referring to him as a “fool,” an “idiot” and a “buffoon,” while comparing him unfavorably to a generation of despotic world leaders that included Mr. Putin and Jair Bolsonaro, the former president of Brazil.

“Democracy has dissolved quickly under our watch,” Mr. Routh wrote, describing the Jan. 6, 2021, attack on the Capitol as a catastrophe “perpetrated by Donald Trump and his undemocratic posse.”