The Doctor, Fitz and Anji find themselves in an Empire where the laws of physics are quite preposterous - nothing can travel faster than the speed of light and time travel is impossible.
Fitz and Anji.
Four worlds in the Empire - Shakrath (pg 42), Thakrash (pg 100), Goronos (pg 180) and an unnamed ash planet with Entropy in overdrive (p210).
None, although the Collectors also appeared in Heart of TARDIS.
Pg 7 Summary of the Earth arc.
Pg 25 "It's something I heard when I spent some time in the mid-sixties" Revolution Man.
Pg 27 "A bit like the more common Vortisaurs" These were mentioned in Storm Warning.
Pg 47 Reference to The Year of Intelligent Tigers, with Anji having met tigers recently.
Pg 63 Reference to the Doctor having lost his memory.
Pg 101 "Then he had lost his memory almost completely" Summary of the Earth arc as seen through Fitz's eyes.
Pg 106 "I have the memories of someone anywhere between a year and several hundreds of years old." Reference to Fitz's experiences of being remembered in Interference.
Pg 118 "Men turned evil by way of evil pacts with the Machine Gods" Possibly the Robomen (Dalek Invasion of Earth).
"Creatures built whole from primeval filth" Primords (Inferno).
"Being made of pure black light" Uncertain reference, but black light featured in The Mysterious Planet.
"A fiery disc which had the power of cutting through mountains to the core of any world of men" This is likely the Time Lord bomb sent to destroy Earth in Interference.
Pg 119 Reference to the Doctor's memory loss.
Pg 123 "You're a Collector." These were in Heart of TARDIS.
Pg 126 "It's just a mercy he was never exposed to The Clangers, I suppose." The Doctor might not have been, but the Master was (possibly as a form of cruel punishment while incarcerated), in The Sea Devils.
Pg 136 (and endnote 18 on page 249) Reference to Year of Intelligent Tigers.
Pg 166 "The freewheeling denizens of the Fractured Planet" This might be a reference to Return to the Fractured Planet, but it might not.
Pg 172 "Standing before the iron door [...] is a dapper , sardonic-looking man dressed in pristine dinner suit. His jet-black hair is slicked neatly back with oil. He is leaning nonchalontly on a silver-handled cane." Also page 192 "Where had he come from? Who was he? Anji had the queasy feeling of her head being invaded by forces quite other than the Cyberdyne itself, and she didn't like the feeling in the slightest." This is Sabbath (on page 172 he says "You know perfectly well who I am") using this opportunity to root around in Anji's mind, even though she hasn't met him yet.
Pg 194 "During his personally discontinuous time in the late sixties on Earth" Revolution Man.
Pg 195 Reference to Enigma devices (The Curse of Fenric).
Pg 219 "It is big, and it is hungry, and it is eating." The Vortex Wraiths are fleeing something in the Vortex. Presumably it's the collapsing multiverse we'll see, starting in Time Zero.
Pg 227 "The roundelled walls were of a uniform white" This is the familiar console room, seen throughout most of the original series.
Pg 237 "I was playing all those bait-and-switch games, thinking how sophisticated I was being and congratulating myself all the while on my cleverness - but, looking back on recent events, I realise how crude my plans actually were." Not-so-subtle dig at a certain former Doctor. Of course, the point is undermined somewhat by the fact that the Doctor has also spent half the book lapsing back into the Congential Idiot; not to mention that none of his plans here were particularly clever, whereas the seventh Doctor's actually were.
Pg 248 Direct reference to Heart of TARDIS.
Pg 249 Reference to Douglas Adams (The Pirate Planet, City of Death, Shada, season 17).
OLD FRIENDS AND OLD ENEMIES
None, although we saw the Collector's race in Heart of TARDIS.
NEW FRIENDS AND NEW ENEMIES
Jamon de la Rocas
Pg 27 Vortex Wraiths.
Goronos, a dead world whose residents are wired into a Virtual Reality machine.
An unnamed ash planet with Entropy in overdrive.