The “promise” of e-books is so old it’s got hair on it, but now two online giants are stepping up to the plate with their own spin on how best to sell books digitally. Next month, Amazon will finally release its long-rumored Kindle, an e-ink reader which will wirelessly connect to Amazon via EVDO, so you can purchase books even more easily than new iPod Touch owners can buy songs while they’re at Starbucks. And before the end of the year, Google will start charging for full online access to some digital copies of books in its database.