What do you mean “guidelines” and “recommendations”, the only Right and Correct way to start reading Discworld is to choose a random book from the library shelf, start there, try to figure out what the hell is going on, embrace the weird and cool concepts and the fact that this setting is just Like That, fall in love with characters, have the best time of your life and repeat (the next book you choose may seem totally unrelated, but it’ll all start coming together after a while)