There's no such thing as a shallow book, only poorly written ones. Thanks for the Pratchett guide, I never know whith which I should start.
Well, If I were to offer up a Pratchett Guide, specifically referring to discworld, then it is probably a little different. The first 2 books are a good introduction to the world. If you enjoy fantasy, then you will get a kick out of those. From there, you primarily have a choice of which "sub-series" you want to go at. There is: The Watch, The Witches, Death, Rincewind, and more recently Moist. There are also some books here and there that actually have nothing to do with any of the subplots, but those are some of the best one, namely Small Gods and Monstrous Regiment. My favorite sub series is The Watch. It has one of my favorite literary characters ever. Following that its probably the witches. Keep in mind all of the books have been good, in my opinon, but some stand out as his best. The Night Watch, Small Gods, Monstrous Regiment, and Wyrd Sisters. Terry Pratchett has an amazing ability to use the medium of satyrical fantasy to say alot about real issues of race, sex, and justice.
And I am just now realizing having typed this that you probably were not asking for a guide to Pratchett, but w/e there it is.