On Friday Apple informed me that they had rejected the TWIS iOS app on the following grounds:
11.13 Apps that link to external mechanisms for purchases or subscriptions to be used in the app, such as a “buy" button that goes to a web site to purchase a digital book, will be rejected.
I have appealed this decision as this provision is intended for in app purchases but the TWIS app does not do any in app purchases it has links to Amazon, iTunes and iBookstore so that the user can easily find the books that are read in the TWIS Book Club.