Über-Chef Thomas Keller will be at Williams-Sonoma in the Mazza Gallerie tomorrow signing copies of his cookbook, Ad Hoc at Home. The event, which begins at 5 p.m., unfortunately penalizes those who may have purchased their cookbook long ago — Keller will only be signing books purchased at Williams-Sonoma. While it may be sound business practice, it’s a shame that this punishes those who were early adopters of the book. For those who are…