What I thought about is to link in the text to web pages, but even simpler would be to store images as footnotes, so that if people read Westertoren and they wonder what it looks like, they can click the word and view a photograph, then hit the back button to resume reading. Also with a map of Amsterdam, so people can see the locations on a map. Shouldn't be too hard to realize in an e-book. With the images stored as footnotes in the digital book, you wouldn't need an internet connection to view the images.