Book creator has probably been the app I have used most, in my teaching, with pupils and in my training. The blank canvas aspect means it can be used across the whole curriculum and the addition of the pen tool in the last few weeks has added to that.
We use Showbie at school for pupils to share their work, including books made with Book Creator from the iPads and home to the teachers for assessment. Recently, we have used both the Pen Tool and Record feature to give feedback on the pupils' eBooks. The pupils send their books using Showbie and the teacher opens them up on his/her iPad. They can then annotate with their voice, pen and text. The book can then be sent back to the pupils using Showbie. The pupil can either change the original book and delete the annotated one or change the annotated book and delete the original.
The screenshot shows a book of a Science experiment. The teacher can annotate with arrows but also add audio feedback. All elements of Book Creator can be deleted so the pupil can restore any annotated book to the original.
This is obviously not a new idea but the pen tool has certainly made this quicker in a widely used app such as Book Creator.
Via iPad Teachers