This is a short book, a quick adventure story that's a reasonably fresh take on the theme of aliens among us. The characters are interesting and the story is engaging. There's no time travel, and the book comes to a satisfactory ending. Really, there's only one thing that I didn't like in this story, and that is that he hangs a fair bit of the detailed plot of the story (though not the main situation and the real core of what's interesting about the story) on an event that seems so improbable that it significantly weakens the whole story for me. I can't talk about it more specifically without revealing something that I don't think you should know at the beginning of the story, but I feel the need to vent about this, so I'm going to go behind a cut tag with some spoilers.
The basic story is that we have one single individual of a powerful alien species on the planet that has special powers that make it able to take any form it wishes and to be pretty much unkillable. That's fine; I'm willing to suspend my disbelief for that. But when Haldeman introduces a second single individual of a different alien species which has rather similar powers to be the villain, the coincidence strains my suspension of disbelief well past the breaking point. No aliens here at all, I can buy. Just one, I can buy. But if there are two, it's very hard to believe that there are only two. And if there are two distinct species, it is very artificial that they're both truly immortal (to the point where reproduction is alien to them) and both have the power to change their shape so they can easily pass for any particular human they choose; it seems entirely contrived for the benefit of having a good fight between the two.
Quite a nice little story, but for one big flaw. 7 out of 10.