I am sure it all made sense inside Tom Holt's head but I found the plot jumps in this book very confusing, and I speak as one who has a number of left-field Robert Rankin novels under his belt.
The basic idea - fictional characters coming to life, and real-life characters entering the world of fiction - is OK, but having come up with the idea Holt does not seem to know where to go with it. The explanations for how people and characters pass across to the other side are strained, and the book has dated badly, having evidently been written in an era when hard disk drives were a rarity (the "modern day" author character in the book uses a floppy disk on which to store her manuscripts).
Jasper Fforde does this sort of thing much better. If you have not yet experienced Tom Holt, don't start with this book. He has written a lot of funny novels, and this one certainly has its quota of laughs, but I found the story very unsatisfying.