There is a lot that I like about this book. The method of time travel in this book is clever and unlike anything I've seen before. (More than 100 naked people are found standing immobile in a cave, one of them is Nick. This is how they travel back in time. Chronologically speaking, they "awake" in that cave in their destination year in the 20th century and walk out of the cave, but a copy of them -- their time trail -- stays in the cave. In the year 2011 they walk into the frozen copy of themselves, and both the moving person and the time trail disappear.) And, the whole idea of the consistent time loop which was done well in the first book is taken to an even higher level here. It really is fun to see how it all comes together.

And, of course, there is some math in the book also. We are frequently reminded that Nick is good at theoretical math (and his friend Simon is good at computation), and this justifies all of the weird stuff involving time and brain science. Nick gets a tattoo of a "closed integral" on his hand. Nick meets with a young researcher who is doing temporal research and offers him both a bribe and advice ("You should look at n-spaces on high -order manifolds of the zeta-mapping. Collaborate with a topologist in your maths department if the equations get away from you. Those guys are all looking for experimental partners.") And, at one point it says that he is working not only with imaginary numbers but also "hypothetical" ones (whatever that means).

A tiny spoiler: All three books involve the protagonists dealing with a real adventure and also playing a game of D&D. They actually spend quite a bit of time (and pages) on this fantastical role playing game. The events in the game, especially those related to the title of the book, are clues to what they should do in their real lives. For instance, the "dispel illusion" spell which Nick must use at one point in the game is a clue that things are not as he (or the reader) believe them to be in the story. Aside: I wonder, would someone whose life is so exciting actually spend much time playing D&D? I thought D&D was for people who found their real lives boring and wanted to escape into fantasy. If in reality you were being hunted by the hired guns of an evil billionaire who unexpectedly found your naked body (and a hundred other people's, including your wife, a famous actress) traveling backwards in time from the future in his underground lair, if your choices were causing paradoxes that threatened the stability of the universe, if the only person you've ever met who is a better mathematician than you is your daughter from an alternative future, etc...would you still want to sit around a table rolling the dice to see how many hit points you lose?