While some public libraries around the country offer digital copies of books, the selections are generally rather limited. But a pair of new services is putting the entire card catalog at the fingertips of tablet and smartphone owners. Both Scribd and Oyster began offering e-book subscription services this month, giving avid readers access to a library of thousands of titles for a small monthly charge. (Scribd asks $9 per month, while Oyster charges $10.) Call it the Netflix-ization of the literary world.