The Bookmark is about half the size of a standard Spellbook page and comes in the colors dark red, dark blue, dark green, and black. Every Bookmark also has a silver or gold tassle. Most also have some sort of rune or arcane script with meaning to the wizard drawn upon them. The Bookmark can be anywhere in the book for its magic to take effect.
The magic of the Boomarks is a protection spell that prevents others from reading your Spellbook. Each Boomark is keyed to the wizard at the time of its creation, you cannot simply purchase one of these at a shop. If the Bookmark is placed inside a book, usually a Spellbook, and anyone other than the Boomark’s owner tries to read the book or remove the Bookmark the magic immediatly takes effect. The magic can be one of many things. The book will: Turn to dust, sand, glitter, or something similar. Instantly liquify. The pages could bond together and the book turn to stone. The book could explode, injuring the person attempting to sneak a peek.
Of course all of these seem to be highly unwanted effects to happen to your spellbook. That is why a command word is keyed to the bookmark at the time of creation that allows you to, when said at the location the book was destroyed, reassemble the book.