I love Nightwatch the best, as well. The Witches stories have never really caught my interest, despite supposedly being amazing. There's just something amazing about Vimes and the night watchmen that I can't let go of. I reread those books at least once a year.

You might also enjoy Monstrous Regiment, Going Postal, and...the second one with Moist. It's about banking. I forget at the moment. Oh, also: Men at Arms. Awesome.