"The Secret Visitors" by James White

The Secret Visitors has serious problems, though in sum I enjoyed the story. It opens with Doctor John Lockhart, a WWII veteran, on a curious British Intelligence mission to prevent an upcoming war. His job is to identify when a mysterious old man is about to die, and to get to him in time for a last minute interrogation. When he does so, the man gibbers in an unknown language, and the Intelligence types seem rather eager to conclude that he is an alien. Before long there seem to be several factions of aliens to deal with, including a beautiful girl, several of these dying old men, and a crew at a hotel in Northern Ireland (not coincidentally, I'm sure, White's home).

The Intelligence people soon make there way to this hotel, and they learn that an evil alien travel agency is fomenting war on Earth in order that the planet, the most beautiful by far in the Galaxy (apparently because it is the only planet with axial tilt!), be maintained conveniently unspoiled for alien tourism. (It should be noted that the aliens generally seem to be fully human -- basically Spanish.) The beautiful girl is trying to smuggle evidence of this perfidy to the Galactic Court in order that the agency can be stopped. For this she needs the help of some humans -- and she seems particularly interested in the help of Lockhart. But is she telling the truth?

This setup is so extravagantly silly as to almost make the book impossible to continue with. And it isn't helped when White can't seem to decide if his method of interstellar travel involves time dilation or not (there's a man from two centuries in the past as a result of one space trip, but on the other hand this impending war seems possible to stop in short order via a round trip to the capitol planet and back.) And there is the absurd bit that Earth's medical science is so advanced compared to the aliens that Lockhart is treated almost like a god. (But the aliens have an immortality treatment -- that, it turns out, for unconvincing reasons, is WHY their medical science stinks.) And there's the part about Earth music being so superior that the aliens are reduced to tears of joy and admiration by an amateur harp player.

Still, there are good parts, such as the alien Grosni, who live partly in hyperspace. Lockhart, in a segment recalling White's Sector General series, must treat a sick Grosni. The story spirals outward from the beginning premise, leading to an action-packed but again not very convincing conclusion, with it must be said a fairly clever final resolution to the final battle. It's by no means a good novel, and I don't think it could possibly sell today, but it is in many places pleasant and imaginative entertainment.  (Strange At Ectaban)