If you are able to read random parts of a book on an as-needed basis, then it's probably a reference book. Of course you don't usually read something like that

If you are able to read random parts of a book on an as-needed basis, then it's probably a reference book. Of course you don't usually read something like that straight through. Most books are difficult to read in random order, though. Maybe you aren't reading many non-reference books.