Exactly, which take quite a bit of getting used to (see my comment above), because it doesn't work that way for real books. In fact when people cite references with page numbers, they usually give the precise editions of the book, since different editions may have the sentence or point being referenced on different pages. So it's important for the "bookmark" to hold the information about the whole text that the user sees a that time, I think.

In case you didn't know, "Locations" are actually sentences, images, etc. I don't know how you can get much more specific than identifying the exact sentence that you'd like to bookmark and remember.

Which makes it impossible to do anything other than read certain books straight through. I wanted to skip around in Infinite Jest, but since the chapter names are meaningless and all the book guides have page number references, it's impossible to know the book's outline.