Left Bank Books, the venerable Central West End bookstore that recently turned 39 "for the first time," in the words of events coordinator Danielle Borsch, held a grand opening celebration at its new downtown location, 321 North Tenth Street, last night.Ben Brantley, the eleven-year-old son of co-owner Kris Kleindienst, welcomed visitors to the store while brandishing a chunk of cheese from the refreshment table. Customers wandered among the bookshelves, chatting and browsing. There was wine and