It's a pretty mediocre story overall featuring the Paternoster Gang.  They get caught up in a whole incident with the Universal Church from Time of the Doctor.  There's parts of A Perfect Christmas that are very good.  I like how the narrative is told from Vastra's perspective.  There's a lot about her character and past as a Silurian that gets glossed over on the show entirely.  So exploring it here a little bit was extremely welcome.  Plus Dave Rubben is very good at slipping in references and making this world feel very connected in a wonderful way.

If I were just going by the story overall, though, I'd say it was pretty basic.  The gang themselves aren't that funny or deep, and the story goes in a pretty straightforward direction.  Sophie Aldred really makes this special, though.  She does an excellent read of this in the audiobook.  There's this cyborg attacking London due to this Urchin stealing what turns out to be a TARDIS key, and the way Aldred voices the cyborg, who is singing little Chrismas lines and hymns as it blasts away, was particularly funny and memorable, and that alone makes this audiobook worth checking out.