I have read every Terry Pratchett Discworld book there is to date.
Thief of Time as a book is great but Thief of Time as a audio book is fantastic. Christopher Cazenove, Gabrielle De Cuir, Karesa McElheny, John Rubinstein, Stefan Rudnicki, and Harlan Ellison are the narrators but what they should be called are the actors. They capture the character's personalities and voices so well they seem real. There is some atmospheric music in the background at key times that is a nice touch. This audio book is definitely worth your money. It takes Terry Pratchett's Discworld books to a whole new level.
Fantastic Audio has a new Terry Pratchett audio book coming out called Night Watch. If Thief of Time is any judge of the quality of their audio books, Night Watch will be well worth your money too.