not sure if you can delete the "att" phone book, but you can try. Here's what I've learned: To remove it, go to Contacts and click Options. Then type rset, it will pop up with a screen asking if you want to wipe the contacts, select yes through all windows. You can then wirelessly sync or use desktop manager to repopulate contacts.
You saved me a big hassle. I had my AT&T address book all screwed up with multiple entries of each contact, so an online AT&T tech deleted the online address book for me. But, I was still stuck with the AT&T contact list on the the phone and the Torch only letting me put new contacts in the ATT Contact List instead of the default book.
I did what you suggested by typing rset and answering YES to deleting the AT&T phone book but answered NO when asked if I wanted to delete all entries in my default address book. It works perfectly now. MANY THANKS!