Diamonds Are Forever, like the movie version of You Only Live Twice, both take the basic plots and settings of their novels, and turn them both into stories about Blofeld using space weapons.
The plot initially appears to be similar. Bond is undercover as Diamond smuggler “Peter Franks” to uncover a diamond smuggling ring, aided by female smuggler Tiffany case, with a trail that ultimately leads to Las Vegas, and is pursued by hitmen Mr. Wint and Mr. Kidd, with a final showdown on a cruise ship.
However there are major differences. In the novel, Bond travels to New York initially to track down the ring; in the movie it’s Amsterdam in Holland. The new York segment in particular utilizes a death by mud bath-in the movie of a diamond smuggler, this scene is still used, but in the pre-credits teaser, where Bond presumably throws his arch-nemesis into the pit of super-heated mud (turns out it’s a body double, made in part using that mud). This is implied to be, in part, revenge for the death of Bond’s newly married spouse, Tracey Bond, in the final scene of the previous movie, On Her Majesty’s Secret Service. (Given that the novels are in a different order, this doesn’t happen for a bit book wise).
“Welcome to hell, Blofeld!”
In the novel, it’s the Spangled Mob, a gang run by the Spang brothers out of Las Vegas. Bond also confronts one brother in a fake “Wild West” town outside Vegas, while he travels directly to Sierra Leone to take care of another, by destroying his helicopter, a kill done by Wint and Kidd in the actual film (and to a different smuggler).
“If god had wanted man to fly….” “…He would’ve given him wings, Mr. Kidd.”
In the movie, it turns out it was Blofeld behind the smuggling ring, using the diamonds to build a giant laser satellite.
In the books, Tiffany Case is mostly a serious character with a tragic past. However in the movie, she’s mainly played for comic relief, as the films were sort of undergoing a transition from the more serious thrillers of the 60’s to more comedic, gadget-laden adventures.