01 Buxton, B., Greenberg, S., Carpendale, S., & Marquardt, N. (2012). Sketching User Experiences: The Workbook. Morgan Kaufmann.
02 Eberle, B. (1996). Scamper: Games for Imagination Development. Prufrock Press, Inc.
The 10 plus 10 exercise  is a great way to get started with a design challenge. Based on a common starting point, group members work individually to quickly sketch several ideas each, making around 10 ideas per group. They share the ideas within the group and choose one sketch as the starting point for the next round. After the second round, there are about 20 sketches per team on the table – a wide range of options from the first round, and a deeper drill from the second. All 20 are useful.
This method helps teams to quickly generate a broad variety of concepts, but also get some depth in understanding how a specific design challenge can be tackled. The visual approach helps them get specific.