Ultima Underworld II Easter Egg - Nystul's Little Secret

In Nystul's library in the castle, you'll find one of the chests is locked and can't be smashed open. One way it is to open it is to leave the castle and enter any of the eight worlds through the blue gem down on the 5th level (doing so will allow you to cast spells in the 4th circle or above).

Cast 'Open' (the cursor will change) and return from the blue gem and back to the castle. Open the chest and you'll find the book Nystul was hiding: 'Sex' by Madonna!

There are plenty of funny other books in the library too, including "Something under the bed is drooling" - a popular 'Calvin and Hobbes' book.