Defining book co-author
A co-author is


  An author who collaborates with another to write something.


There is nothing in the definition that states that each author must have written literally half of the book. Obviously, if there are more than two co-authors (perhaps more common for articles, your question seems to indicate that you realize this), they cannot all have written exactly half of the book, and it is rather unlikely that each will have written exactly one-third or one-fourth or...

In your situation, if the original/first author offers you co-authorship, accepting is a judgment call on your part. You have authored many books; is this one worthy of being added to your list? On the other hand, if you are not offered co-authorship, but will be acknowledged otherwise for your contribution, it is probably best to accept this with good grace. Next time--assuming that you might collaborate again--be sure to clarify potential authorship roles before beginning the actual work.