You can turn this off by an option in the MOBI Output settings.
You can also tell calibre whether to put it and the start or the end of the book via an option in the MOBI Output settings. The fact that it is part of the text in MOBI is an accident caused by the limitations of MOBI.
For example, in the calibre viewer, you use the Show Table of Contents button to see this To C. This is because the MOBI format, alone amongst mainstream e-book formats, content To C at the end of the book.
However, the Kindle firmware tends to malfunction if you disable the generation of the end-of-file inline To C.
We’ll be on the lookout for show-stopping bugs or battery drain issues but we don’t expect to see anything like that.
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As always, battery drain for i OS 10 is likely going to be higher on the first day of use.
i OS 10 does a lot of indexing and caching and these all get flushed on system update. Let us know what you think of Apple’s latest mobile operating system in the comments.