Many additional books can be found using Quick Search and any of the following searches:
soldier* AND register*
soldier* AND biography
This will bring back a LOT of results. You may want to restrict your search to look for these terms "in the subject" - - using the Advanced Search (linked on right side of search box or lower right below the box) and/or by adding in additional geographic terms (such as Iowa or France) or the name of a specific war.