In The Blood, by Jenny T. Colgan

Second paragraph of third chapter:

Donna dropped her suitcase and opened her arms. Hettie was standing in her pristine Chiswick doorway. She lived in one of the posh houses, down by the riverside.

Jenny Colgan is clearly among those who regard the Tenth Doctor / Donna pairing as one of the high points of New Who; as well as this novel, she wrote a Big Finish audio starring Tennant and Tate earlier this year. This is both rollicking sparking adventure with the two protagonists rubbing along beautifully (plus of course Donna’s grandfather Wilf), and also a sombre reflection on why everyone is so nasty to each other online these days; finding the answer takes the Tardis crew to Korea and Brazil (incidentally places where Doctor Who has cult status) to track down the alien force responsible (if only it were just aliens rather than human nature). I was fortunate to read a particularly scary chapter set on a plane immediately after disembarking from a tediously delayed flight rather than during it. But this is glorious stuff. (I see some reviewers chiding that internet trolling was not as bad in 2008 as it is now, and some specific name-checks are anachronistic; but I can live with that – there are many bigger anachronisms in the Whoniverse.)