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This is the Eighth novel in the series and was released in 1973. The novels were a series of adventures featuring the titular character (MI6A's special agent Richard Blade), who was teleported into a random alternate dimension at the beginning of each novel and forced to rely on his wits and strength. Along the way, he would have several explicitly described sexual encounters with beautiful women (both in England and in the alternate dimensions), and would usually return from his adventure with some item, or bit of knowledge useful to Britain (the ostensible reason for him being sent in the first place). Richard Blade was distinctly British, and all the stories are set in England (at least at the beginning and end, with Blade being teleported to some other dimension for the bulk of each tale).
Used availability for Jeffrey Lord's Undying World
June 1973 : USA Mass Market Paperback
1973 : USA Mass Market Paperback
December 2004 : USA Audio CD
March 2004 : USA Audio Cassette