Margi, you should be able to add 'Discworld' yourself. When the spellchecker underlines it, right click your mouse and you'll get a pop up with suggestions and an option to "Add to dictionary".
I always have to teach my computers all about Discworld as they don't have a clue.
I'm so glad you are enjoying Discworld. I envy you that you have so many to look forward to reading for the first time.
Hah! Dunno why I haven't thought of that before. You might remember, that some time in the past I complained that my spell checker always underlines "you". Not anymore.
P.S.: It also underlines "anymore"; that's spelt correctly, isn't it?