You can export the text data out of an iBooks e-book by using the save as.. or export function within the program when the original file is opened.
Convert iBooks e-book to Simple text file.
We have found one software suitable in our database for this conversion.
Apple macOS / Mac OS X software - convert ibooks to txt on OS X
Apple's free authoring tool for making great looking e-books for iPads
Convert from ibooks file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.