I would think that when you picked the book up the system would determine if there actually was an item in the book MOST items people use as bookmarks are very thin and stuck aginst the spine and in most cases just shaking the book woulden't dislodge the item especially in old book becuase the pages tend to be thicker and stick together more, so the only way to actually FIND the item is to turn every page and read the whole book.

I don't see a coin being used as a bookmark but I could see one of the cards from the card system or maybe even small parts of a spell scroll, or if we used paper money in Elanthis then i would see money as a rare treasure.