"While this sounds innocent enough, it isn't. If signed by an author, the letter is actually a contract amendment that will materially and adversely affect the royalty rates of many Bloomberg Press authors."
The alert explains contractual changes, and advises, about a letter sent to Bloomberg authors: "The Authors Guild strongly urges Bloomberg Press authors to not sign this letter without careful consideration. If you have received this letter, consult your agent or a publishing attorney or contact a lawyer in our legal department so you understand precisely how this amendment would affect your rights and royalties. Important: if you have already signed the letter and returned it to Wiley, contact our legal department immediately. Non-Guild members are welcome to contact us as well. All communications will, of course, be held in confidence."
Read the full text of Authors Guild notice
Read the Wiley letter (PDF)
Wiley responded, and the Guild's responded to that response in Turow to Wiley: "Knock it off and do the right thing."
http://authorsguild.org/advocacy/articles/wiley.html