D You most definitely do not need to read this book to go on in the series, this particular cheap trick to squeeze some cash from us don't add anything to the overall, and very good, storyline. Maybe the 2 last chapters can be interesting, but not nearly important.
Katrina I agree that it's not "necessary", though that's also hard to answer not having read the next book. You learn a lot more about the Obin and a few gaps in the plot of The Last Colony are filled in.
Greg Zoe's tale stands alone fairly well, though there is no question that it is 'within' the Old Man's War series. It's a parallel narrative of the Last Colony, which I now really need to read (missed that one!)
Liz If you read The Last Colony you don't need to read this at all, except the few scenes of the other residents and interactions away from home. It gives a synopsis of the previous book, so it might stand alone and would be a worthy YA book, given the teenager perspective and the lack of swearing and sex.
Travis Knight No, it's 3.5 in the series; the entire plot takes place within the confines of book 3 and only fills out two or three scenes in addition to what the (superior) previous entry does.
Erin Brenner I disagree. You don't have to have read the whole series, but you do need to have read the previous book, The Last Colony, which is source of Zoe's Tale. In Last Colony, the colony's story is told from John Perry's point of view and is the fuller story. Zoe's Tale fills in gaps, as Katrina noted, but Zoe's point of view is very limited (much as a child's or teen's POV would be in the real world) and without reading The Last Colony the story can be confusing.