Rummaging skeletons gives a 1/13 chance of receiving an ancient page, while the spirits and skeletons have a 5/128 (1/25.6) chance of dropping them. In addition, mithril dragons have a 1/64 chance of dropping them. Brutal green dragons do not drop the pages. They are part of the book "my notes". There are 26 different pages. Because they are uncommon drops, and the fact duplicate pages can be obtained, my notes may take several hours to complete. A good strategy on claiming ancient pages is to rummage the three skeletons found on the lower level, and hop to another world.