Well...
Did you send the book as an email attachment? If so, check your sent email for the attachment.

How easy this is to fix depends.
Is the book still in Word format?
Has it been converted to another format and then perhaps converted back?
How many Word sections are in the book?

A three-inch line length is not unusual in a book, especially with smaller type sizes. Longer lines are hard to read.

Bottom line: ALWAYS keep backups of important documents. Always keep backups of documents you send to someone else. (That's the lawyer as well as the computer nerd in me speaking.)

If in Word format, you may want to take the book to a Windows environment and use the following Add-In which only works in Windows Word:
Word Toolkit for Advanced Management of Word Headers and Footers

The title of that should be management of sections but most people would use it for headers and footers. Margins are also a section property.
Sections / Headers and Footers in Microsoft Word (Ribbon Versions)

That toolkit would allow you to look at each section in the book and make changes.

Are you sure you are talking about margins and not indents?
Margins and Indents in Word
If every time you wanted to change the distance from the edge of the page (as for a quotation) you changed the margin you are going to have a ton of extra sections.