Hello - not sure how 'complex' your book may be? Of course, this will impact on the features you may need. But, Apple offers your a free set of office type apps, i.e. Pages, Numbers, and Keynote - Pages is the word processor - more information
HERE - plus, the app offers plenty of templates, including a number for 'books' - see pic below from my iPad and note the two rows of 'books' which can be scrolled for more selections. Plus, there are plenty of options for saving and sharing your documents - do a practice doc and then explore the choices to determine if one or more serves your needs. Dave
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