This book started as a script for TV written by Douglas Adams. The script does not follow what we think of as Doctor Who cannon. Production did start on an episode (or movie?) but (apparently) not completed. Gareth Roberts took the script and expanded it into a book.
The book is an adventure story, it seems to be based on the Tom Baker version of The Doctor, but it isn’t explicitly stated. It does have a good feel of Doctor Who, the deviations are pretty obvious and appear early, and it is easy to get past them and enjoy the story. The flavor of Douglas Adams is obvious, especially toward the end of the book.
The characters are interesting, although the relationship between the two primary human characters felt contrived. The plot is interesting and has enough twists to keep the interest up.