I am looking for a book series that upon completion will leave a feeling of emptiness, as if I'm saying goodbye to a good old friend, when you realize the good old days were left behind. It's a shitty feeling that only comes after something great, something that left a mark on you.
The Discworld novels by Terry Pratchett are humorous, well written and quite engaging Once you have a few under your belt, you get to appreciate the wit, kindness, and empathy of the author. GNU Terry Pratchett.
Ps Start with Going Postal or Small Gods and then do whatever you want from there. Search for Discworld reading order for more info