Sitemaps can be a great way to let search engines know when you have published new material. Google provides a lot of data on sitemaps through their Google Webmaster Central interface.
Every time you update your data be sure to update the sitemap. Google allows you to submit your sitemap as often as you like.
One area that often irritates webmasters is the way Google presents the data. Google may report that the sitemap has been read and that x pages out y have been indexed.
A simple check will reveal how many pages are actually indexed. The feedback from Google Webmaster Central is really telling you how many new pages were indexed.