So my internal chronology order reading of the PDAs and Telos novellas featuring the Seventh Doctor starts with this 2002 novel, set immediately after Battlefield, involving alien invasions, UNIT and an old people's home. There is a decently complex alien behind it all, and a nifty bit of timey-wimey manoeuvring at the end, but I felt it was fairly average. The Seventh Doctor / Ace relationship is nicely reset to the Season 25 status, this after years of development through the Virgin New Adventures. It was Michalowski's first novel - I've enjoyed his later work more.
Oddly enough I was reading this at the same time as listening to the new Torchwood audio, Forgotten Lives by Emma Reeves, which is also set in an old people's home but is much better.
The next Seventh Doctor book in internal chronology is The Hollow Men by Keith Topping and Martin Day, but I read it in 2010, so I will skip ahead to Dave Stone's Telos novella Citadel of Dreams.