Tuval claims Zygor was destroyed when the Zygons enemies, the Xaranti caused a stellar explosion, probably by destroying its sun.
The HADS (Hostile Action Displacement System) is mentioned.
The State of Temporal Grace is mentioned as the Doctor has repaired it, preventing the Skarasen from acting aggressively.
The way the Doctor changes the subject when asked about Sam in the epilogue implies that this occurs during the period they are separated between Longest Day and Seeing I; the latter novel also implies this.
The Doctor is still searching for a copy of The Strand in the next novel, Genocide.