Good books on modulation are hard to come by. I would suggest taking a look at some more general harmony books such as Schoenberg's Theory of Harmony or the texts by Kostka and Payne. They can be fairly expensive so I would try to get my hands on one from the library before shelling out the bucks, just in case it isn't what you are looking for. In general a good book on classical harmony should cover modulation, I think Tchaikovsky's book on harmony is pretty good. You might also look at jazz harmony for another take on how to modulate.
Hope that helps.
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