Terry Pratchett books are to be frank, better than most other books. Maybe his first books were really just average but his worst Discworld book*
is still probably well worth reading and a cut above the rest.
Thief of Time is one of the best Discworld books.
It's quite hard to describe why. It's fun, constantly moving, full of tiny interesting details, everything is a set-up, a reflection on a theme, an interesting event and a twist all at once. It feels smart reading it. Thief Of Time
is in a kind of transition period between the old and newer books. Instead of being about fundamentally real world things it's about the wise monks in orange robes who control the universe and have martial arts powers tropes, but it still mixes that in with the themes and ideas about what humans fundamentally are that are found in the later Discworld novels.
For instance Lu-Tze is a little old monk in a valley high up in the mountains who decided if everyone is coming to them to find enlightenment, he'd travel to the decadent modern city and find perplexity. It feels like a joke, but it never needs to be a joke because you're also being told a serious genuine story, it's just everything the story is made at can be examined and shows this funny little quirk. But you don't have to get out your glasses to understand the story, instead you just read it and enjoy it. It's like instead of eating bread with all the holes and air, suddenly you're eating steak.
There is very little wrong with this book. The characters are interesting, the pace is good, it doesn't feel like it dithers, which maybe the earlier books could do a bit and there is thing after thing to be discovered. I don't want to describe it too much because everything in it is better when you find it out yourself.
This is probably a very good book to start with, the style gives you an experience of the earlier books and the later ones, but there's very little continuity here, everythings understandable by itself and it has very little affect on the other stories, but lots of little references that will make those stories feel more familiar. If you haven't tried Discworld before or you tried the first few and didn't like them, try this book or Going Postal