Doctor Who and the Arc of Infinity (in-universe)

 was a book depicting the series events of when the Fifth Doctor and Nyssa reunited with Tegan in Amsterdam, and then had a "dreadful affair" with Omega trying to take over the Doctor's body. It was set after Doctor Who's Time Flight, when the Fifth Doctor left Tegan at Heathrow Airport, and Doctor Who and the Land of the Dead, though that hadn't always been the case. (PROSE: Bafflement and Devotion)