The clue is in the name. This book has one of those plot structures that looks pretty from the outside, but is rather confused on the inside. A huge derelict spaceship with apparently some strange control over hyperspace and time-travel is at the core of a war between two neighbouring powers (isn't that always the way?).
The Doctor and Sam thread their way through this, but for once don't save the day; after all it's not possible to detangle the loops to save the day. The book ends on an unusually downbeat note, but that makes it a good book rather than a mediocre one I think.