CK-12 Study Now! is a free iPad app from the Ck-12 Foundation. The free iPad app puts CK-12 Flexbooks (multimedia books) on students’ iPads. Students can browse the app by subject and topic. Each Flexbook has to be downloaded individually to students’ iPads.
In some cases CK-12 Flexbooks can be a good alternative to buying textbooks. In other cases they make good supplementary material for your lessons. Either way, CK-12 Study Now! can help you and your students find helpful reference material.
As others have reported, I found the app to be crash-prone. Hopefully, this will be fixed soon.