The overall writing gets a B+. It flows almost all of the time, but this is not the stuff of Pulitzer Prizes, nor is it intended to be. There is no eloquent prose, and there are a few typos.
Effort to please the fans and stick to the canon also gets a B. It's really a shame it's not an A+, because heroic efforts were obviously made to tie up all the loose ends in a Mobius strip of words. However, it is also obvious is that no hard core fans were invited to proof read for errors of content. In addition to reprinting the errors from the Prologue to Pursuit, there were several new mistakes in the back story, which leave the devoted and the uninitiated both asking, 'why bother?' These include: (1) the reappearance of a gun that was permanently melted on the television show in part of a brilliant bit of cinematography, (2) mention of Tess using a car bomb (instead of her alien powers as actually occurred on the show) to destroy a military base, and (3) agent John Stevens, who died in the first season of the show, is credited with disbanding the Special Unit in season 2 (it was really done by the shapeshifter Nasedo posing as agent Pierce).
It would have been better if more space had been devoted to explaining the mechanics and biology of the scifi elements (which were, by the way, quite cool) instead of doing a back story rewrite that was not quite accurate. Admittedly, the back story was nicely woven into the plot, rather than taking up chunks of pages, which the reader would likely skip. But Roswellians (as fans are self-monikered) are sticklers for detail and accuracy.
Still, what really bothers me about Turnabout is the horror factor (or 'yech' factor, as in the X-files). References to gore are not without precedence in either the tv show or the original Roswell High series. However, the emphasis here is at the expense of the relationship building which was the foundation of the fan base.
Die hard fans will want to read Turnabout, although, to paraphrase Nicholas from the Harvest episode, the series may be over, but surely Turnabout is not the end of the story.