If you want to set the search index to all you can use the keywords parameter to search for items.
If you want to search for a title and author you need to set the search index parameter to Books, or some other category that supports these - it is explained in the documentation:
www.spiralscripts.co.uk/Documentation/am...n-documentation.html
Look at the part that explains the searchIndex - the available search Indexes are listed, and it is clearly stated that you need to use Books in order to do an author or title search.
An example is
{amazonWS:searchIndex=Books;title=Harry,Potter}