Update: The book has been identified! It's "The Game of X" by Robert Sheckley. Highly recommended.

Thank you to @DziadekMick for the correct title, to the person who DMed me a link to a digital verson, and to all who boosted.

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I'm trying to find a book I read forty years ago. I think it had "X" in the title, but I'm not sure. It is NOT related to an apparently very popular series of books about "Secret Agent X" that infests every internet search I attempt :-(

It is about a man who is NOT a secret agent, but who is mistaken for one and thrust into ever more desperate situations as a result. As the book progresses he hears about (and must perforce demonstrate) an ever increasingly improbable set of talents he is supposed to have (but really, REALLY doesn't). Nonetheless luck and coincidence let him survive against the odds and he begins to suspect by the end of the book that somehow he may actually BE the chap everyone seems to think he is.

There is an episode involving a ridiculously high-powered motorboat that would stick in your memory; it had me in literal tears of laughter. The last chapter involves a fight with axes and other nasty bladed weapons, also memorable.

If you have read this book you will remember it and if you remember the title or author I would love to hear from you!

Please boost for reach :-)

PS: It's also not related to the James Garner movie "A Man Could Get Killed" (nor the book it's based on) which has a sort of similar device, but nothing like the same plot.