Back in the 1950's and 60's, Ace books published as a sci-fi series a number of "Ace Doubles" - books with two front covers, upside down relative to each other, and two novellas/novels you could read back to front. Those doubles usually contained works by two authors, although some were two stories by the same writer, and sometimes there were two stories by the same author but one published under a pseudonym. Robert Silverberg had some of his early work presented in such a manner, and tells you all about those heady days in a nice little introduction. Then its off to the republication of three 50 year old 90-odd page stories (its not 400 pages all up, as the desciption says) in a strictly conventional one after the other fashion. In fact, I have no idea how one *could* do an "Ace triple", save by using non-euclidian geometry...but I digress. How do these old stories hold up? Pretty well.
The Plot Against Earth is an adventure story with aliens, spaceships and blasters, but not hard sci-fi in the sense there is no lovingly detailed description of how things work: there is faster than light travel (nullspace) and matter duplicators but this is just by the by, like hopping on a bus would be to you and I. It's a mystery story as well, and while I can't say the plot twists are any great surprise the plot moves fast enough to hold your interest to the end.
The titular story smacks of Minority Report (which it post-dates by a year or two) but instead of "pre-cogs" we have a computer prediction of a future war that is best averted by destroying a planet now, before it can cause the deaths of 50 billion, rather than just 3 billion. A secret agent is sent to this world to carry out its destruction with planet killing devices, and finds a horrible world seemingly without hope of redemption. But there are some small details that raise doubts here and there: and who wants 3 billion deaths on their conscience? This is more than just an run of the mill story, dealing with issues as well as action, and reasonably well done, too.
The last story is "One of our asteroids is missing" which really should be titled "MY asteroid is missing". Its about a young space-miner who lays claim to a mineral-rich asteroid, only to find upon his return to Earth that his claim has not been recorded...and then that he has been deleted from all computer records. Something is up, and he has to go back to his claim to find out what...and its nothing he was prepared for. This is a nice little romp, reminiscent of Brackett in a way, almost a Western in spirit. Its amusing to see that the book is set around 2018, and that there are private spacecraft and that there has been a Mars colony since 1970-odd. Obviously, things did not turn out like that, but you wonder if they could have, in a slightly different world. There a few plot holes when you stop and think about it, but nothing that slows you down when reading the story.
All up this is a pretty good little collection, fun to read and well written enough to make you think from time to time.