The post-finale "Legacy" series has been one of the better examples of this tie-in novel strategy.  Other shows have used novels (or, more recently, comics) to finish out or otherwise extend the story beyond the televised adventures.  The example that most resembles the "Legacy" concept, in my mind, is the "Roswell" set of novels that wrapped up that show's mythology.

"Legacy" has been most successful in its approach to the characters.  Over the course of the four books already released (out of a total of six), the vast majority of the page count has been devoted to in-depth character study and evolution of relationships.  That's not to say that there hasn't been plenty of action; it's just that the authors have been more focused on the characters than the actual episodes tended to be.  The characters are actually changing, growing, and evolving.

It's important to keep that in mind, because despite bringing the second act of "Legacy" more or less to a close, "The Furies" is remarkably low on action.  The very end has a stirring action sequence, and there's a big moment in the first half, but more than 90% of the book is devoted to characters talking to one another.  Enjoyment of this novel is going to depend greatly on one's patience for that sort of material, especially when entire chapters roll by with conversations and E-mails to off-page characters that merely allude to a larger context.

For fans of the entire franchise, particularly "SG-1" and "Atlantis" and those focused on certain possible romantic relationships, this is all gravy.  But it does mean that the pacing is sometimes slow and erratic, and there are times when it feels like there's no clear direction to the narrative.  One gets the sense of pieces moving on the board, but the game plan isn't apparent.  This is, by far, the least focused of the "Legacy" novels to date.

The highlight of the novel would have to be Teyla's plot thread, which ties back into her character's history in several important ways.  It's also connected to the ongoing exploration of Todd/Guide's character, and by extension, the deeper exploration of the Wraith, which has been a highlight of the "Legacy" books.  All too often, giving too much context to the enemy can undermine their effectiveness; this is not the case for the Wraith in "Legacy".

There is more than enough here to sustain the interest of any die-hard "Stargate: Atlantis" fan, and since one is likely to be that devoted if they are looking for further adventures, then this is highly recommended for that audience.  Casual fans may find that it meanders a bit too much for their taste, and references enough of the "Stargate" franchise's history to make some connections and thought processes a bit confusing.  But to be honest, I doubt too many casual readers would be picking this up in the first place.