I really liked the initial Attar the Merman adventure, ATTAR'S REVENGE, and the second one, WAR OF NERVES, also published in 1975, may be even better. Unfortunately, Joe Haldeman, the acclaimed science fiction author who hated writing them under the name Robert Graham, gave up the series after these two quickies.
Attar was reared on an island off the coast of Australia where lies Aquatic Research Associates, Limited, a foundation operated by marine biologist Wallace Hamilton and funded by millionaire John Tucker. An expert in linguistics and mortal combat, Attar is able to breathe underwater, thanks to Hamilton's operation that gave him (and his late sister Essence) gills.
Attar's mission this time is to save the Atlantic Ocean from destruction at the hands of a madman named Rasputin, who has discovered a cache of nerve gas dumped in the Caribbean and plans to explode a bomb and disperse the gas unless the United States pays a ransom. On the case is Attar, as well as his brother Victor (Tucker adopted many children at birth and trained them similarly to Attar, though only Attar has gills) and his pet killer whale Grampus. And if that isn't awesome enough, know that Attar and Grampus share a telepathic bond so that they can "speak" to each other.
WAR OF NERVES is a great, pulpy read with an exciting plot and intriguing characters. It has plenty of action and violence, though nothing too graphic. The relationship between Attar and Grampus is weird, but seems normal within Haldeman's framework. The author adds tension to their relationship when Grampus rescues Attar at sea by grabbing the young man's leg with his mouth and discovers he likes the taste of human flesh.
Haldeman also tosses in a twist involving the plot's true villain and a pretty sweet finale involving Attar and his party's invasion of the baddie's Haitian stronghold. Both Attar the Merman adventures are fun, brisk, and exciting, and it's a shame Pocket Books never did others. Considering Haldeman wrote them under a pseudonym and prolific packager Lyle Kenyon Engel, one wonders why they didn't continue with a new author.