If 'Timewyrm: Revelations' was Paul Cornell ringing in the New Adventures, 'Love and War' is the point where he buries the old TV series. Appropriately enough, he does so in a novel that's all about death and mourning. The funereal atmosphere permeates the book from the opening chapter to the closing epilogue (both of which feature actual funerals), but more than that, the book is about the death of a friendship. Ace and the Doctor's relationship, as we have known it in the 17 stories so far, ends here in a story that's about as close as Doctor Who will ever come to pure tragedy.
Allow me to elaborate on that for a moment, because I think it's key to understanding the novel: Tragedy, in its dramatic sense, is all about someone attempting to avert an inevitable, devastating event...only to find that their efforts come to nothing because their essential nature contributes to the event instead of preventing it. The Doctor knows, arguably from within minutes of stepping out of the TARDIS, what the danger is and how to stop it. It's the simplest solution in the world, requiring just one death, and saving untold billions as a result. And the Doctor spends the entire novel trying to figure out a way to make it not happen, because he knows that Ace will hate him for it...only to find that, in the end, he has done everything except for the one thing that he should have done from the beginning. He hasn't told Ace the truth. He hasn't trusted her. Trust, the one thing that the Seventh Doctor endlessly demands, is the one thing that he cannot give. That's his fatal flaw. And his act of hamartia is to tell Ace to stay away from Jan, to warn her that she will be hurt if she gives the Traveler her heart, without ever giving her the reason why.
It makes sense, in retrospect, why he would conceal this from her. The Hoothi have eyes and ears everywhere, and Ace is a woman who wears her heart on her sleeve. Giving away the secret, even to Ace, could result in the deaths of billions. (We also find out later that this is part of a Bigger Plan, an element that Cornell downplays as much as possible and one that badly weakens the story, but that's Peter Darvill-Evans' fault, not Cornell's.) But we aren't looking at this from the Doctor's perspective. His efforts are sidelined in the novel, occurring on the margins while the book focuses on Ace. We don't see inside his head the way we did in 'Revelation'; instead, he becomes an opaque, ambiguous figure. We never get an explanation from him. We never hear about the things that led to this failure to save Jan, leaving us in Ace's position of wondering whether he ever truly wanted to or if that was just another aspect of his game. By concealing that from the reader as well as from Ace, Cornell makes the Doctor into the cause of his own reversal of fortune. His flaws as much as the Hoothi's evil lead to the tragedy he desperately wants to avert, leaving him wounded and alone.
Save, of course, for Bernice Summerfield, the new companion who is distinctly a creation of the books. Her relationship with the Doctor is already more complex than the blind loyalty that hurt Ace so badly, and it's a sign that the past is over. 'Love and War' buries the TV series, and in so doing finishes the process that Cornell began five books ago. From now on, we are in (should you pardon the pun) novel territory.