The Soto uke protects the Chūdan level, the middle area of the body. 

It directs the attacks with the help of a circular movement the attack to the side, from the outside to the inside.

Soto uke rarely appears in the Kata. However, as a combination, it already appears in the Heian nidan. The technique is also used in the Master Kata Unsu.

You can find the matching book here:
The Essence of Karate