ePubChecker™ is a free app to check your EPUB files for correct format.
To publish your Apple iBook, they require your manuscript be verified by the 'epubcheck' checking engine.
ePubChecker also harnesses version 0.1.0 of the "epubpreflight" engine to see if your book follows suggestions to give the best results for your readers.
You can find other tools on the Web for checking your file. ePubChecker has a few advantages:
Privacy: Your manuscript stays on your computer. Other tools upload your book to their computers.
Easy to use: Just drag and drop your .epub file.
Fast: Working locally on your own computer is quicker than repeatedly uploading.