Below are some links to some important information regarding importing into the United States.
Importing into the United States: A Guide for Commercial Importers
Overview of some of the processes and requirements of importing into the US. This is especially good for new importers to read
China Section 301 Tariffs and Exclusions
Use this site to try to stay updated on the lists and exclusions as they evolve
Customs Rulings Online Search System (CROSS)
Use keywords to search through rulings made by CBP to help determine your HTS number
CBP Informed Compliance Publications
These are put out by US CBP as additional information into how items are classified