So, it sounds like you mostly have intangible assets.
The books should be covered by copyright law - the term for copywrite is long (in the US). You may want to check CA laws. Not sure if you only publish, or do print as well. You may have copyright to any cover art as well.
You could model how much you make on each book, how long sales last, how many books you are projected to publish in the next 1, 2, and/or 5 years - and generate some values from that (may want to report as a range - expect to generate between $x - $y in sales).
If there are books in a series, I would expect the value to be different than 15 independent books (the series will boost sales of the older books, create more interest in the new).
You may also want to describe what you do to protect and enforce your copyrights.