In an Internet-based textbook, an author can quickly add new information, making it immediately available to all readers.
Some texts are free, but usually the user pays a fee of some form.
Most commercial sites provide a free trial period. They often add other related resources not available in the paper version. Prices are dropping as more physicians purchase this format. The paper textbook has been a useful medical information tool for centuries. This format now faces competition from the computer-based formats that provide more rapid access to information.