Seattle may be home to Amazon. But any Northwesterner will tell you the best bookstore is Portland-based Powell’s. The independent bookstore is in no danger of going out of business. Indeed, in spite of the general corporatization of the industry, Powell’s has flourished, making major expansions to its store in recent years. Now it’s bought a 60,000 square-foot warehouse about two miles from its downtown location to handle its expanding online business. The new building will store one million used books.