As the three top textbook publishers -- Pearson, McGraw-Hill and HMH -- move slowly into the iPad textbook market, several questions remain about growth in the sector. Teachers using digital textbooks on iPads say they and their students are enthusiastic. Some, however, question whether the size of the textbooks is too large for iPads and whether they take up too much bandwidth. Some also question whether the digital textbooks' reliance on streaming could be an issue during an outage.