Here's a podcast from Gerry Rodrigues of Tower 26 where he talks about periodization (periodization stuff starts about 10 minutes in):
http://tower26radio.libsyn.com/episode-1-introduction-to-tower-26-radio-and-the-5-pillars-to-building-a-successful-swim-training-plan
http://tower26radio.libsyn.com/episode-1-introduction-to-tower-26-radio-and-the-5-pillars-to-building-a-successful-swim-training-plan