The novel Summer Of My German Soldier deals heavily in the ideas of race, ethnicity, belonging, and loving the inside of a person, not their outer appearance. To that degree, a really good thesis statement may involved the use of one or more of those themes.
The best idea is to locate passages that will effectively illuminate the point that you are trying to make. Since the above mentioned themes are the most prevalent, the list below gives you a good basis to find textual evidence for your argument. Also, remember that you thesis statement should not be an undefined and nondefinitive opinion of the story.
Love conquers fear.
Love is stronger than prejudice.
The need to belong is stronger than the need for freedom.
Self acceptance is the greatest gift to own.
Love destroys hatred.
Individuals are much more than they appear.