Tim, you picked one of the best (and most philosophically challenging) books to start with. Nicely done.
Glad you liked it!
Hmm -- Carpe Jugulum (for me, at least) is a very high point in the "Witches" cycle. If you're planning to read CJ, I'd advise you to start with "Wyrd Sisters", because it introduces most of the witches and, at least, one of the main characters in Carpe Jugulum -- and it's also very funny, to boot.
I'm sure some people will tell you to start from "Equal Rites", since it introduces Granny Weatherwax, one of the most formidable witches you'll ever meet -- and also Eskarina Smith, a very fun and strong character!
But in my not-so-humble opinion, it's not necessary to read "Equal Rites" in order to understand "Carpe Jugulum". On the other hand, "Wyrd Sisters" is a must.
"Interesting Times" isn't the best book to start with. ("Interesting Times" was my first book, and I was sometimes baffled by who Rincewind and Cohen were).
It's probably easier to start from either "The Colour of Magic" or "The Light Fantastic" - they were written as a pair (TLF being a sequel to TCOM).
Happy reading!