Hopefully you have found the answer by now, but if not, you can do the following.
The item can be set as TX, NT, or you can add user defined categories in Inventory Item Maintenance. Once this is done, you go to Modules, Librar Master, Common and setup tax codes (i.e. State, County, City or whatever) and tax schedules (which combine multiple jurisdictions to arrive at a total tax amount by tax class)