Resolution Centerを見てみると
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We found that your app is primarily a book and is therefore not appropriate for the App Store, as noted in the App Store Review Guidelines.
Books should be submitted to the iBookstore. To work with Apple on iBookstore distribution, first verify that your content meets the following requirements:
- ISBNs for all titles you intend to distribute
- Is in EPUB format, passing EpubCheck 1.0.5
To obtain an ISBN for your content and learn more about ISBNs, please see US ISBN Agency. Then complete the iTunes Connect Online Application.
Visit iBookstore Aggregators for information on working with the iBookstore aggregators.
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こんな感じのメッセージが。
要するに、単純なブックアプリはIBOOK STOREで売れってことらしいです。
そもそもiBookstoreは日本語対応していないのでその理由もどーかと思いますが、
とりあえずこちらの言い分を書いて再審査の結果を待ちます。
電子書籍アプリ作成：電子書籍制作キット