Select the book you want to synchronize and open it. Select itunes to open when opening, so that you can find that there are book options in itunes at this time.

Try to synchronize in the ibook under the application and find the ibook. Itunes prompts that this program supports document transfer, just drag the file to the document list under ibook, but ibook does not support TXT, only supports pdf and epub. Pay attention to the file format.