Generally, the chefs do book signings after the seminars. The key is to get to EPCOT right away in the morning, go to the festival center, and get your wristband. That's kind of like a "fast pass" for the signing - all those with wristbands (in my experience) will get their books signed and they have a disney cast member who will take a picture with your camera with the chefs during the signing. If you don't have a wristband, you aren't guaranteed to get your book signed. Similarly, even if you go to the seminar, you aren't guaranteed to get your book signed. The wrist bands are key!
Also, as a tip, we pre buy the books from
amazon and bring the books with us - much cheaper than buying at Disney!