Specifically, I'm thinking about binary. Converting to binary shows up in a lot of standards. "We" think it's an important fundamental of computer science. Yet for most students - and many teachers, I'd wager - there's no connection to computer science. We tell them that computers work in binary, but then they use web browsers and interpreters and word processors and there's nary a 0 or 1 to be found.
Wouldn't it be awesome if there was some kind of book that put all these things into context? There are certainly more examples. Maybe if you comment with some of the ones that occur to you, someone will put them into a book.