As it is, Amazon's and other companies "recommendations" are generally unappealing to me. If Amazon having all this information will help make recommendations that really am interested in, then I'll gladly give out these kinds of publicly available and generally harmless data. So long as they don't share the information and they provide easy ways for me erase the data that I give them. If I were to buy a book about woman's health for my girlfriend, I wouldn't want 100 "recommendation" asking me to buy other feminine books, especially when I've clearly marked that I'm male. Long story short: This database is only acceptable if actually works.