The Shack restaurant in East Sacramento was packed Saturday, the overflow filling the patio. So many people turned up for the book launch of photographer Tom Myers’ “Postcard History Series: Sacramento,” that he sold the 65 copies he'd brought in little over an hour.   The book is a visual history of Sacramento, taken from Myers’ collection of vintage postcards and supplemented with specific descriptions and historical details.   Myers began collecting postcards about six years ago as an extension of his interest in the history of the Sacramento area. His collection has grown to more than 680 postcards, most of them spanning the years 1905 to 1940.   The introduction to the book was