I'm not sure about "best", but I got a lot out of Will Pirkle's two books on programming synthesizer / effect plugins (http://www.willpirkle.com/about/books/).
For a more generic guide I've heard a lot of good things about a free (in electronic form) book called DSPGuide (http://dspguide.com/). Haven't had a chance to dive into this one, though.
For a more generic guide I've heard a lot of good things about a free (in electronic form) book called DSPGuide (http://dspguide.com/). Haven't had a chance to dive into this one, though.