Create a phone tree of all your book club members, with you at the top of the tree. Put two people below you, for whom you are responsible when it comes time to cancel. Put two people below them, and so on. Include the phone numbers on the phone tree.
Pass out the phone tree at the first meeting of your book club. At your first meeting, pass around a sign-up sheet to get everyone's email address.
Call the two people underneath you on the phone tree when you need to cancel the book club. Tell them the specific date you are canceling, or if you are dissolving the club altogether. Those two people will then contact their two people, and so on until every member is aware of the cancellation.
Send a mass email to the addresses you collected at the first meeting, if the phone tree isn't working. You can also send the email as a confirmation of the cancellation.