User error--Apparently Rincewind can only hold up to 2 items at a time (aside from skills). Once I moved the book to the luggage, I was able to move the butterfly net into his inventory.
When playing the Discworld CD-Rom with ScummVM on PC, there seems to be an issue with putting items into Rincewind's inventory. As quoted from another user on another forum: "I have three things in it now, pouch, book and pickpocket skill. I couldn't get the mirror in it so I just carried it to...