I got an email from a reader yesterday wondering about an error that iTunes Producer was giving as it rejected his EPUB submission to the iBookstore (despite having passed EpubCheck with nary a murmur), to wit:
ERROR ITMS-9000: "Invalid URI in NCX file Title Page.html :
Illegal character in path at index 5: Title Page.html" at Book
When he converted the spaces to underscores, the errors went away and iTunes Producer accepted his book.
His book was originally created in InDesign, and it was InDesign that failed to convert the spaces to their hex equivalents so that iTunes Producer wouldn't choke.
Until Adobe updates its software, and for everyone who codes on their own or with some other tool, I recommend restricting filenames to the 26 unaccented letters of the English alphabet, the dash (-), and the underscore (_). That includes the names of documents in an InDesign book, images, videos, audio, CSS files you apply upon export, whatever.
Note that you can have spaces in titles, say Chapter 1, just not in filenames of those chapters, e.g., chapter1.xhtml.
A quick search on Mobile Read reveals that this is not a new issue.
Thanks so much to Edward Matte who brought this to my attention and shared his error reports and progress with me (and all of us).