I'm not sure what the Harvard referencing system demands, but in the UK referencing of the sort you mention is:
1) Where there are two successive footnotes or endnotes referring to the same work, for the second you put "ibid."
2) Where there is more than one reference to the same work, but they aren't successive, you put "op cit."
Hence if you are referencing two books - "How To Do It" by J Jones and "What To Do" by T. Smith, the references would look something like this:
1) Jones, J, "How To Do It", London 2004, p 44
2) Smith, T., "What To Do", New York 1998, p 158
3) ibid., p 163
4) Jones, op. cit., p51