My experience is that the dimmer in iBooks is the actual dimmer for the device (iPhone or iPad). So, when I dim the iBook and then exit the book, the dim setting is as set in the app.
So, yes, it goes very dim for reading, but on exit, that setting is still that way.
Sorry that doesn't solve your problem, but thanks for mentioning it, because now I know NOT to use the iBook dimmer.
Marilyn
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